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11.04: Refractometer

  • Page ID
    3238
  • The refractometer is one of the most important tools in a gemological laboratory. It indicates (not measures) the refraction index of a gemstone, which often gives vital clues to the identity of a gemstone.

    Although one would expect a refractometer to measure the refraction of light inside a gemstone, this is not the case. Instead it is based on a unique optical phenomenon named Total Internal Reflection (or TIR).

    For a better understanding of the refractometer, you first need to understand refraction.

    Basic

    Construction of a gemological refractometer

    File:Refractometer.jpg

    Figure \(\PageIndex{1}\): Cross section of a standard gemological refractometer
    (modified image from an Eickhorst SR 0.005 refractometer)


    Light (1) enters through the rear of the refractometer through an opening (1a) in (or before) which a yellow sodium filter can be placed. It then hits a mirror (2) which transmits the light to the center of the hemicylinder (3).
    This hemicylinder is made of high refractive glass (usually N-LaSF by Schott with a refractive index of ~ 1.88 at nD and a hardness of about 6.5 on Moh's scale).
    At the boundary between the hemicylinder and the gemstone (4), the light will be partially refracted inside the stone and partially reflected in the hemicylinder (see below on Total Internal reflection). The reflected rays (5) will pass through a reading scale (6) and a lens (7) or a series of lenses, depending on the type of refractometer.
    The reflected rays hit a mirror (8) which directs the light to the ocular (9) and then outside the refractometer to your eye (11).
    The ocular (9) can slide in and out for better focus and is usually accompanied by a detachable polarizing filter (10).

    As the hemicylinder has a relatively low hardness compared to most gemstones, care must be taken not to scratch it. That would ruin your refractometer, as optical contact between the gemstone and the cylinder would be impossible and would give you false readings.

    Total Internal Reflection

    Figure \(\PageIndex{2}\): Inside the refractometer: Total Internal Reflection

    When light travels from an optically denser material (with higher index of refraction) to an optically rarer material (with lower index of refraction), all light that reaches the boundary of the two materials will be either reflected inside the denser material or refracted into the rarer material, depending on the angle of incidence of the light.

    For every two media in contact in which light is traveling from the denser to the rarer medium, the dividing line where either the ray of light is totally reflected or refracted is fixed and can be calculated. This dividing line is named the critical angle (ca). On the left you find an image showing the critical angle as the red line.
    When light reaches the boundary of the two materials at an angle larger than this critical angle (the blue line), the ray of light will be totally reflected back into the denser material. Light reaching the boundary at an angle smaller than the critical angle will be refracted out of the denser medium (and a small amount will be reflected) into the rarer medium (the green line). All light traveling precisely on the critical angle will follow the path of the boundary between the two materials.

    Note

    In the example above, the light seems to come from 3 light sources, but the principle is the same when coming from a single point.

    In a hemicylinder, the incident and exiting ray always reach the boundary at a 90-degree angle when directed to the center. Refraction doesn't occur when a light ray is at 90 degrees to the boundary. A hemicylinder is used so there will be no refraction of the light entering or leaving the denser material.

    The standard gemological refractometer can make use of this phenomenon because the reflected rays of light will appear as a light area on the scale, whilst the refracted rays are not visible (and therefore appear black). The light/dark boundary shown on the scale of the refractometer is a visible representation of the critical angle.
    The standard gemological refractometer thus measures the critical angle between the glass hemicylinder and the gemstone and plots that on a calibrated scale. This type of refractometer is hence better named a "critical angle refractometer".

    Lighting

    Proper lighting is one of the key features when using the refractometer.

    Although one can get results using a white light source, the standard is monochromatic yellow light with a wavelength of about 589.3nm. This light source is historically used as it was easily produced by burning table salt in a candle (at a very low cost). All gemological refraction indices are based on the use of sodium light (or nD). For more information, see Fraunhofer.

    The use of different wavelengths can produce different readings. As the refractive indices of gemstones are measured with an accuracy of 0.001 decimal, sodium light should be used. All gemological tables of refractive indices are produced using this light unless otherwise stated.

    White light may be used for single refractive gemstones or to obtain a first impression. One should look for the boundary between the green and the yellow of the allochromatic white light source.

    However, for double refractive gemstones, one should then switch to a sodium light source, simply for the reason that the double refraction readings in white light may easily overlap and it would be impossible to get a correct reading. And of course the boundary between the lighter and darker areas is better defined, making the reading easier to take.

    Always buy a refractometer with either a sodium filter or a sodium light source.

    Contact liquids

    Here things get a bit more complicated.

    Contact liquids are used to create an optical contact between the hemicylinder and the gemstone. This is to prevent air from trapping between the facet of the stone and the hemicylinder, which would ruin the Total Internal Reflection effect.

    As this contact liquid also has it's own refractive index, there will also be Total Internal Reflection between the hemicylinder and the liquid. It is important to ensure that the tiniest drop of liquid is used so the stone doesn't float on the liquid. Use just enough to create a "thin film". Donald Hoover added to this through personal communication that too much liquid will not only lift up the stone slightly, the reading may also be off slightly due to the refraction inside the liquid (the ray will deviate slightly). With a thin film, this is marginal and will have little to none effect on the reading.

    The result is obviously two Total Internal Reflection readings, one from the hemicylinder-liquid and the other from the liquid-stone boundary (which will be, due to laws of refraction, the same as if no liquid were used). That is the reason you will also see a faint reading near the higher index of the scale on the refractometer, which is the reading of the liquid.

    The refractive index of the liquid sets the limit of which stones can be tested on the refractometer. Usually, the liquid has a refractive index of 1.79, but some have a refractive index of 1.81. You can not measure stones that have a RI higher than the liquid used. Stones with a higher RI than the liquid will give you a "negative reading".

    Liquids with higher RI are available, but they are so toxic that they are only used in specially equipped laboratories. They would, of course, also need a special hemicylinder which will be of higher RI than the liquid.

    You should always shield your contact liquids from light (especially for the 1.81 type) and care should be taken not to let the liquids crystallize.

    The chemical compositions of the liquids are:

    • 1.79 - Saturated solution of sulfur and di-idiomethane
    • 1.81 - Saturated solution of sulfur, di-idiomethane and tetraidioethylene

    Always wash your hands after you make physical contact with the liquids -- not only for the smell.

    Use of the Refractometer

    Video \(\PageIndex{1}\): Video showing how to use a refractometer

    As with every instrument, success depends on proper usage.


    First, you apply a very small drop of contact liquid on the center of the hemicylinder of the refractometer, after which you place the stone you want to investigate table down next to the hemicylinder. With your fingernail, slide the stone on the center of the hemicylinder. For an oval stone, place it lengthwise.

    At this point, the contact liquid will suck under the facet and provide an optical contact between the stone and the hemicylinder. Do not apply any pressure to the stone by pushing it down on the cylinder as that would damage the hemicylinder. (Repairs are very costly.) Close the lid of the refractometer to shield the stone from any surrounding light. Remove the polarizing filter if it hasn't been removed already.

    Now, with the light source in place at the back, place your best eye (usually your right one) just before the ocular of the refractometer. You should position your eye so that you look at a straight angle to the ocular, to prevent a "parallax error". The best way to know your eye is in the right position is if you can see the whole scale (or most of it) without moving your eye.

    Now find the dividing line between light and dark on the scale. (For gemstones cut en-cabochon, the technique is slightly different. See the "distant vision" method below.) If the scale seems blurry, you can slide the ocular in and out for better focus. Now you can start taking your readings (explained below).

    When you are finished, gently slide the stone off the hemicylinder and remove the stone with your fingers if possible. It is important to keep the hemicylinder clean, so use a clean cloth or tissue to gently wipe any remaining contact liquid from the cylinder. Do this gently without any pressure, making a North-South motion.

    As mentioned above, the hemicylinder is made of a relatively low hardness glass and can easily scratch. So always make sure you keep abrasive materials and sharp objects (like tweezers) away from the hemicylinder.

    Look at the images below to see how to properly use the refractometer.

    Click images to enlarge

    File:Rf1.jpg

    Figure \(\PageIndex{3}\): Open liquid bottle and get small drop

    File:Rf2.jpg

    Figure \(\PageIndex{4}\): Carefully place on middle of hemicylinder

    File:Rf3.jpg

    Figure \(\PageIndex{5}\): Drop should be no larger than this!

    File:Rf4.jpg

    Figure \(\PageIndex{6}\): Place stone parallel to length of hemicylinder

    N.B: Some people find it hard to get a small drop of liquid directly from the bottle. A different technique is to place a series of small drops (usually 2 or 3) next to the hemicylinder and place the stone on the smallest drop, then slide the stone and liquid together onto the hemicylinder. Alternatively, one can lose excess liquid from the liquid rod by making a few drops next to the hemicylinder and then apply the remainder directly onto the refractometer's hemicylinder. Whichever method one prefers will work.

    File:Refractometerscale1.jpg

    Figure \(\PageIndex{7}\): 1.544

    We notate refractometer readings to a precision of 0.001 (one thousandths). The refractometer scale has subdivision indicators to 0.01 (one hundredths). Between the two horizontal bars which indicate the 0.01, you will need to estimate the final precision.

    In the image on the right, you will see that the shadow edge is between the 1.54 and the 1.55 bars. Between these two values, we need to find the last precision. As it is just above the middle, the last precision is 0.004. So the reading is 1.544.

    Estimating the last decimal needs some practice. Some refractometers, like the Eickhorst ones, have a more detailed division of the scales which makes taking a reading easier. With a little experience, you will find an easier-to-read scale is not needed.

    Faceted gemstones

    Following is the method for taking RI readings that is used for faceted gemstones. En-cabochon and sphere cut gemstones require a somewhat different technique which is explained in the "distant vision" section.

    Figure \(\PageIndex{8}\): Starting position
    1st reading

    Figure \(\PageIndex{9}\): 45-degree rotation
    2nd reading

    Figure \(\PageIndex{10}\): 90-degree rotation
    3rd reading

    Figure \(\PageIndex{11}\): 135-degree rotation
    4th reading

    When taking refractometer readings, one usually starts with the largest facet (which is usually the table facet). Place your stone in the starting position, then close the lid of the refractometer. Make sure the light source is on.

    Position your eye in front of the ocular in a way so that it is at a straight angle with the refractometer scale. You will now most likely see a dark region at the top of the scale and a lighter region in the lower part. If you have chosen a monochromatic sodium light source, there will be a sharp line between the lighter and darker areas. That line is named the "shadow edge". (You may also observe 2 less sharp "shadow edges".)

    Place the polarization filter on the ocular and, while looking at the scale, turn the polarizer 90 degrees left and right. You will observe either of two possibilities:

    1. only one shadow edge is seen
      • the stone is either isotropic or
      • the incident light reaches the stone at an angle parallel to the optic axis and you should turn the stone 90 degrees
    2. you see the shadow edge move between two values on the scale
      • the stone is uniaxial or
      • the stone is biaxial
    • In the first case, where only one shadow edge is seen, the reading for the shadow edge will remain constant during a 135-degree rotation of the stone. For every rotation reading, take two measurements: one with the polarizing filter in North-South position and one with the polarizing filter in East-West position.

    The readings in the images below indicate a single refractive (isotropic) stone with RI = 1.527, which is most likely glass. (If one finds a single refractive transparent faceted stone with an RI between 1.50 and 1.70, it is most likely glass). Taking four sets of readings (with the polarizer in both positions) on a single refractive stone looks like overkill, which it is; take them anyway.

    First reading Second reading Third reading Fourth reading

    File:Refractometerscale5.jpg

    1.527

    File:Refractometerscale5.jpg

    1.527

    File:Refractometerscale5.jpg

    1.527

    File:Refractometerscale5.jpg

    1.527

    File:Refractometerscale5.jpg

    1.527

    File:Refractometerscale5.jpg

    1.527

    File:Refractometerscale5.jpg

    1.527

    File:Refractometerscale5.jpg

    1.527

    • In the second case, where the shadow edge moves between two values on the scale, write down both values you see, in table form below each other.

    Below are 4 sets of readings of a double refractive stone with a uniaxial optic character (where one reading value remains constant). For every set of readings, you rotate the stone 45 degrees with your fingers without applying pressure while leaving the stone in contact with the hemicylinder.

    First reading Second reading Third reading Fourth reading

    File:Refractometerscale1.jpg

    1.544 ω

    File:Refractometerscale2.jpg

    1.553 ε

    File:Refractometerscale1.jpg

    1.544 ω

    File:Refractometerscale3.jpg

    1.552 ε

    File:Refractometerscale1.jpg

    1.544 ω

    File:Refractometerscale4.jpg

    1.549 ε

    File:Refractometerscale1.jpg

    1.544 ω

    File:Refractometerscale3.jpg

    1.552 ε

    1st 2nd 3rd 4th
    lower readings ω 1.544 1.544 1.544 1.544
    higher readings ε 1.553 1.552 1.549 1.552

    While taking your refractometer readings, write down the values you read on the scale. For every set of readings, the polarization filter is turned 90 degrees. In addition to this, you can also take a fifth reading (180-degree rotation).

    In the example above, the lower readings (1.544) stay constant while the higher readings vary. In other gemstones, the higher value may remain constant while the lower value changes.

    Note

    The lower reading is the reading of lower value, not lower on the scale.

    The RI of this stone is 1.544 - 1.553 (smallest lower reading and largest higher reading). This indicates quartz.

    To calculate the birefringence of the gemstone being tested, you take the maximum difference between the largest higher reading and the smallest lower reading. In this example, that is 1.553 - 1.544 = 0.009 .

    Some gemstones have a lower reading that falls within the range of the refractometer (and the liquid), while the higher reading falls outside the range. Those gemstones will give you just one reading on the refractometer and should not be confused with isotropic gemstones.

    • Gemstones may also have two variable lower and higher readings, but the procedure remains the same. You write down the lower and higher readings in a table and calculate the birefringence.

    First reading Second reading Third reading Fourth reading

    File:Refractometerscale6.jpg

    1.613

    File:Refractometerscale7.jpg

    1.619

    File:Refractometerscale8.jpg

    1.611 α

    File:Refractometerscale10.jpg

    1.616

    File:Refractometerscale11.jpg

    1.614

    File:Refractometerscale7.jpg

    1.619

    File:Refractometerscale8.jpg

    1.611 α

    File:Refractometerscale9.jpg

    1.620 γ

    These readings give a biaxial reading with RI = 1.611-1.620 and a birefringence of 0.009, indicating topaz.

    1st 2nd 3rd 4th difference
    lower readings 1.613 1.611 1.614 1.611 0.003
    higher readings 1.619 1.616 1.619 1.620 0.004


    You may have noticed some odd looking letters in the image footers, like α, γ, ε, and ω (and β which will be seen later on). They are not typos but Greek letters whose meanings will become apparent in the discussion on optical sign. You will also learn why we added the "difference" in the biaxial table.

    Optical character

    Optical character refers to how rays of light travel in gemstones (or most other materials).
    In uniaxial and biaxial materials, the incoming light will be polarized in two (uniaxial) or three (biaxial) vibrational directions which all travel at different speeds inside the gemstone. This is due to the molecular packing inside the stone. For a better understanding, we refer to the discussion on double refraction.

    Gemstones are divided into three categories (characters) depending on the way a ray of light behaves as it passes through the stone:

    1. isotropic
    2. uniaxial
    3. biaxial
    • Isotropic stones are stones in which light travels in all directions at equal speed.
    Among those stones are the ones that form in the cubic system as well as amorphous stones, like glass.
    • On the refractometer, you will see one constant reading.
    • Uniaxial means that light travels differently in two directions.
    One ray of light will vibrate in the horizontal plane, which we call the ordinary ray (ω). The other will vibrate in a vertical plane along the c-axis and is called the extra-ordinary ray (ε). This extra-ordinary ray is also the optic axis (the axis along which light behaves as if being isotropic).
    Gemstones that are uniaxial by nature belong to the tetragonal, hexagonal and trigonal crystal systems.
    • You will see one constant and one variable reading on the refractometer.
    • Biaxial gemstones split up incoming light into two rays as well; however, the crystallographic directions are labeled as the α, γ, and β rays. The two rays both act as extra-ordinary rays.
    Stones with a biaxial optic character have two optic axes.
    The orthorhombic, monoclinic and triclinic crystal systems are biaxial.
    • This will be shown by two variable readings on the refractometer.

    Spot readings (distant vision method)

    This is the method used to estimate the RI of en-cabochon cut gemstones.

    You place a very small drop of contact liquid on the hemicylinder and place the stone on the drop, on it's most convex side (as in the image below). Remove the polarization filter (if not already done) and close the lid.

    File:Spotreading3.jpg

    Figure \(\PageIndex{12}\)

    Move your head back about 30 cm from the ocular and look straight to the scale. On the scale, you'll see a reflection of the contact liquid droplet. When you move your head slightly in a "yes-movement", you'll observe the droplet move over the scale. Try to fixate the point where half of the droplet is dark and the other half is bright.

    File:Spotreading2.jpg

    Figure \(\PageIndex{13}\)

    The image above shows three stages while moving your head. The top droplet is too light and the bottom one is too dark. The one in the center shows a good half dark/half bright droplet.

    Now move your head toward the ocular and estimate the Refractive Index. Unlike with faceted gemstones, we estimate to a 0.01 precision when using this method. The image below shows the reflection of the liquid which is half bright/half dark at 1.54. This gemstone may be Amber.

    File:Spotreading4.jpg

    Figure \(\PageIndex{14}\)

    Alas, one cannot determine birefringence using this method, unless the birefringence is quite large (as with the carbonates). The "birefringence blink" or "carbonate blink" technique makes use of a larger drop of contact liquid and a polarizing plate. As the plate is rotated, the spot will be seen to blink. A crude estimation of birefringence can be made by this technique.

    Advanced

    Optical sign

    Optic sign in birefringent gemstones is shown as either a plus (+) or a minus (-). The reasons why some stone have a positive sign and others a negative sign lies in the orientation of molecules inside the gemstone. This is explained by the use of an indicatrix in the refraction section.

    Isotropic gemstones do not have an optical sign. Light travels at the same speed in all directions.

    Uniaxial stones may have either a positive (+) optical sign or a negative (-) one.
    We calculate the optic sign by deducting the ordinary ray (ω) from the extra-ordinary ray (ε). So in the case of Quartz with ε = 1.553 and ω = 1.544 that will give us a positive number of 0.009. Hence the optical sign is positive.
    A full refractometer result for quartz will therefore be "RI = 1.553-1.544 uniaxial +" and a birefringence of 0.009.

    In uniaxial gemstones, the constant reading is always the ordinary ray (ω).

    If the ordinary ray is the higher reading in a gemstone (as in the case of Scapolite), there will be a negative optical sign. For instance if you have the following readings: ε = 1.549 and ω = 1.560, the calculation will be 1.549 - 1.560 = -0.011 (so a negative).
    This is how we separate Quartz from Scapolite most of the time, the first is uniaxial +, the latter is uniaxial -.

    Biaxial gemstones can also be either positive or negative for the same reasons; however, biaxial minerals have three values that correspond with the crystallographic axes. These are the α (Greek letter alpha), β (Greek letter beta) and γ (Greek letter gamma).
    The indicatrix of biaxial materials is somewhat more complex than the uniaxial one.

    In practice, we are not concerned with the intermediate β value, merely with the higher and lower readings we find on the refractometer. As shown previous, we take 4 sets of readings for every orientation of the stone (0 degrees, 45 degrees, 90 degrees, and 135 degrees). If we put the readings in a nice table, we can calculate whether the higher or the lower readings vary the most.

    1st 2nd 3rd 4th difference
    lower readings α 1.613 1.611 1.614 1.611 0.003
    higher readings γ 1.619 1.616 1.619 1.620 0.004

    As can be seen in the table on the right, the higher readings vary the most (0.004) opposed to the lower readings (0.003), this indicates a positive sign. If the lower reading would have varied the most it would have been biaxial negative.
    So for this Topaz, the full reading would be: "RI= 1.611-1.620 biaxial +" of course we also mention the birefringence as "DR = 0.009".


    As a word of caution, the explanation above is a crude method as the β value has not been determined. When there is doubt about the identity of the gemstone due to the optic sign, make sure you determine the true value of β (here it could be either 1.614 or 1.616). When the polarizer is used properly, one will find that true β is at 1.614 for this stone.

    Overview of the crystal systems

    Structure

    Structure type
    Crystal axes
    Angles

    Symmetry
    (of highest crystal class)

    Optic
    character

    Refractive index
    (RI)

    Optic sign

    Pleochroism

    Gem
    examples

    Amorphous

    No order

    No axes

    No symmetry

    Isotropic

    Singly refractive

    1 RI
    n
    None None

    Glass

    Amber

    Cubic

    Isometric: 1 axis length

    a1 = a2 = a3

    All at 90°

    13 planes

    9 axes

    Center

    Isotropic

    Singly refractive

    1 RI
    n
    None None

    Diamond

    Spinel

    Garnet

    Tetragonal

    Dimetric: 2 axis lengths

    a1 = a2 ≠ c

    All at 90°

    5 planes

    5 axes

    Center

    Anisotropic

    Doubly refractive

    Uniaxial

    2 RIs

    nw and ne

    + = ne > nw

    – = ne < nw

    May be dichroic Zircon
    Hexagonal

    Dimetric: 2 axis lengths

    a1 = a2 = a3 ≠ c

    a axes at 60°;

    c axis at 90° to their plane

    7 planes

    7 axes

    Center

    Anisotropic

    Doubly refractive

    Uniaxial

    2 RIs

    nw and ne

    + = ne > nw

    – = ne < nw

    May be dichroic

    Beryl

    Apatite

    Trigonal

    Dimetric: 2 axis lengths

    a1 = a2 = a3 ≠ c

    a axes at 60°;

    c axis at 90° to their plane

    3 planes

    4 axes

    Center

    Anisotropic

    Doubly refractive

    Uniaxial

    2 RIs

    nw and ne

    + = ne > nw

    – = ne < nw

    May be dichroic

    Corundum

    Quartz

    Tourmaline

    Orthorhombic

    Trimetric: 3 axis lengths

    a ≠ b ≠ c

    All at 90°

    3 planes

    3 axes

    Center

    Anisotropic

    Doubly refractive

    Biaxial

    3 RIs

    na, nb, ng

    + = nb closer to na

    – = nb closer to ng

    ± = nb midway between na & ng

    May be trichroic

    Topaz

    Zoisite

    Olivine (peridot)

    Monoclinic

    Trimetric: 3 axis lengths

    a ≠ b ≠ c

    2 axes at 90°;

    1 axis oblique

    1 axis

    1 plane

    Center

    Anisotropic

    Doubly refractive

    Biaxial

    3 RIs

    na, nb, ng

    + = nb closer to na

    – = nb closer to ng

    ± = nb midway between na & ng

    May be trichroic

    Orthoclase

    Spodumene

    Triclinic

    Trimetric: 3 axis lengths

    a ≠ b ≠ c

    all axes oblique

    No planes

    No axes

    Center

    Anisotropic

    Doubly refractive

    Biaxial

    3 RIs

    na, nb, ng

    + = nb closer to na

    – = nb closer to ng

    ± = nb midway between na & ng

    May be trichroic

    Axinite

    Labradorite

    Optic character/sign with the Refractometer

    Optic character/curve variations: Uniaxial or biaxial

    1. Two constant curves = Uniaxial

    2. Two variable curves = Biaxial

    3. One constant/one variable which meet = Uniaxial

    4. One constant/one variable which don’t meet:

    Check the polaroid angle of the constant curve

    a. Biaxial = polaroid angle of constant curve = 90°

    b. Uniaxial = polaroid angle of constant curve ≠ 90°

    Optic sign

    Uniaxial stones

    1. High RI curve varies = (+)

    2. Low RI curve varies = (-)

    3. Both curves constant: At 0° polaroid angle, only the o-ray is seen

    a. If low curve is seen = (+)

    a. If high curve is seen = (-)

    Biaxial stones

    1. If nb is closer to na, the gem is (+)

    2. If nb is closer to ng, the gem is (-)

    3. If nb is halfway between na and ng, the gem is (±)

    4. If two possible betas exist, false beta will have a polaroid angle equal to 90°. True beta will have a polaroid angle unequal to 90°.

    Polaroid angle

    • 0° polaroid angle is when the polarization axis of light transmitted through the plate is parallel to the refractometer scale divisions.
    • 90° polaroid angle is when the polarization axis of light transmitted through the plate is perpendicular to the refractometer scale divisions.

    Symbols

    Uniaxial crystals

    • nw = omega, the constant RI of a uniaxial crystal
    • ne = epsilon, the variable RI of a uniaxial crystal

    Biaxial crystals

    • na = alpha, the lowest RI of a biaxial crystal
    • nb = beta, the intermediate RI of a biaxial crystal
    • ng = gamma, the highest RI of a biaxial crystal

    Bright line technique

    In some cases, you may find it very hard to get a clear boundary between light and dark using conventional refractometer techniques. In those rare cases you may find it useful to illuminate from the top of the hemicylinder instead of from below.

    Cover up the illumination opening at the rear of the refractometer and open the lid. Place the stone in position as usual and illuminate the stone/hemicylinder in a way that the light is grazing over the surface of the hemicylinder.
    This will give you a very bright area when you look through the ocular and/or a very bright line showing the RI value. This technique is best carried out in a dark environment with a light source that is pointed from the back of the stone (in the direction of the observer). The junction of the stone's facets should be perpendicular to the length axis of the hemicylinder.
    With some practice, this will give you a 0.001 precision.

    When allochromatic white light is used, one can determine the relative dispersion of the gemstone as well as absorption lines in some cases.

    Kerez effect

    Some green tourmalines may show up to 8 shadow edges (tourmaline is uniaxial and should only show two shadow edges in one reading). This is to current knowledge due to heat and/or thermal shock while polishing the table facets.
    Little documentation on this subject is at hand.

    Peter Read added the following in personal correspondence:
    "The effect in green tourmaline was first reported in 1967 by R. K. Mitchell [ed.: Journal of Gemmology Vol. 10, 194 (1967)] and the name 'Kerez effect' was suggested by him. Work on the effect has since been carried out by Schiffmann and Prof. H. Bank. In GEMS, the effect first appeared in the 5th edition and was inserted in Chapter 6 (Topaz & Tourmaline) by the late Robert Kammerling former Director of Identification & Research, GIA Gem Trade Laboratory, USA. I understand that the effect is mainly caused by thermal shock due to polishing, and not to chemical constituents."

    Dietrich [1985] mentions that the highest of these readings (lowest on the scale) are the correct ones.

    This phenomenon was named after C.J. Kerez.

    Different types of refractometers

    A word of caution to all neophyte gemologists on buying a refractometer. Nowadays inexpensive refractometers are offered on the internet for as low as USD 100.00. They are mostly fabricated in China and one shouldn't expect too much from them. Especially obtaining an RI for small and en-cabochon cut stones may prove to be difficult.
    Some sellers put their own respected company logo on them and pass them on as the best your money can buy.
    Always test your new refractometer with a small stone with a known refractive index and make sure it is precise at 0.001.

    Although the price is very tempting, a good refractometer is more costly but will last a lifetime when handled with care.
    Some of them are outlined below.

    The GemPro refractometer

    GemPro refractometers are direct view type refractometers just like the duplex II that GIA makes. Direct view refractometers have removable eyepiece lenses that enable spot reading of cabochons. Other type refractometers can't do this well because they have a different prism design. The eyepiece used with the GemPro refractometer is a special achromatic lens that gives excellent resolution when birefringence and other readings are being observed. The hemicylinders are made of a special German glass made by Schott glass company. These hemicylinders are tough to scratch and resistant to chemicals. Tarnish from the air does not happen with this type of glass. Supplied with monochromatic filter, RI liquid, and MagLight.

    The Rayner Dialdex refractometer

    This refractometer differs from most TIR refractometers that it doesn't have an internal scale to read the values from. Instead, you will see a "window" with a bright area. By turning a "wheel" on the side of the refractometer, a vertical black band will appear which should be lined up with the lower edge of the bright area. After this one takes the reading from the calibrated wheel.
    An external light source should be used.

    The Duplex refractometer

    Made in the USA, this refractometer has an extra large window of view. Making it easier to find shadows.
    No built-in light source, an external one should be used.

    The Eickhorst refractometer

    In contrast to most refractometers, the Eickhorst refractometers have a calibrated scale with 0.005 precision (opposed to the usual 0.01) and this makes estimating the third decimal easier.
    Eickhorst also offers gemology modules of great quality and appealing appearance. Some models have an internal light source.

    The Topcon refractometer

    This refractometer is made in Japan. Very sturdy metal case and made to last. It is one of the most expensive refractometers on the market.
    No internal light source.

    The Kruess refractometer

    Kruess is a long-established German manufacturer of all sorts of refractometers (not only for gemological purposes). Their line in excellent gemological refractometers includes portable and standard ones, with or without built-in lightning.

    Refractive Index of Common Gem Minerals

    Some of the values listed below reflect values which are extreme possibilities for the gem.
    In other words, highs and lows which are, but rarely, seen.
    Remember to always check values for birefringence, as it can be as diagnostic as RI.

    Gem Mineral Refractive Index Birefringence
    Actinolite 1.614 - 1.655 0.022 - 0.026
    Adventurine (Quartz) 1.544 - 1.553 0.009
    Agate 1.535 - 1.539 0.004
    Air (as a point of interest) 1.0003
    Albite (Feldspar) 1.527 - 1.538 0.011
    Alexandrite 1.745 - 1.759 0.009 - 0.010
    Allanite 1.640 - 1.828 0.013 - 0.036
    Almandine (Garnet) 1.775 - 1.830
    Amazonite (Feldspar) 1.514 - 1.539 0.008 - 0.010
    Amber 1.539 - 1.545
    Amblygonite 1.578 - 1.612 0.020 - 0.021
    Amethyst 1.544 - 1.533 0.009
    Ametrine 1.544 - 1.553 0.009
    Anatase 2.488 - 2.564 0.046 - 0.067
    Andalusite 1.627 - 1.650 0.007 - 0.011
    Andesine (Feldspar) 1.543 - 1.551 0.008
    Andradite (Garnet) 1.880 - 1.940
    Angelsite 1.877 - 1.894 0.017
    Anorthite (Feldspar) 1.577 - 1.590 0.013
    Apatite 1.628 - 1.650 0.001 - 0.013
    Apophyllite 1.530 - 1.540 0.001 or less
    Aquamarine (Beryl) 1.567 - 1.590 0.005 - 0.007
    Aragonite 1.530 - 1.685 0.155
    Augelite 1.574 - 1.588 0.014 - 0.020
    Axinite 1.672 - 1.694 0.010 - 0.012
    Azurite 1.720 - 1.850 0.110
    Barite 1.636 - 1.648 0.012
    Bastnäsite 1.717 - 1.818
    Benitoite 1.757 - 1.804 0.047
    Beryl 1.563 - 1.620 0.004 - 0.009
    Beryllonite 1.552 - 1.562 0.009
    Bixbite (Beryl) 1.568 - 1.572 0.004 - 0.008
    Boracite 1.658 - 1.673 0.024
    Brazilianite 1.602 - 1.625 0.019 - 0.021
    Bronzite 1.665 - 1.703 0.015
    Bytownite (Feldspar) 1.561 - 1.570 0.009
    Calcite 1.486 - 1.740 0.172 - 0.190
    Carnelian 1.535 - 1.539 0.004
    Cassiterite 1.995 - 2.095 0.098
    Celestite 1.619 - 1.635 0.009 - 0.012
    Cerussite 1.803 - 2.078 0.274
    Chalcedony 1.535 - 1.539 0.004
    Chrome Diopside 1.668 - 1.702 0.028
    Chrysoberyl 1.740 - 1.777 0.008 - 0.012
    Chrysocolla 1.575 - 1.635 0.023 - 0.040
    Chrysoprase 1.535 - 1.539 0.004
    Citrine 1.544 - 1.553 0.009
    Clinozoisite 1.670 - 1.734 0.028 - 0.041
    Colemanite 1.586 - 1.614 0.028
    Coral 1.550 - 1.580 0.160
    Crocoite 2.290 - 2.660 0.270
    Cubic Zirconia 2.170
    Cuprite 2.848
    Danburite 1.627 - 1.639 0.006 - 0.008
    Datolite 1.621 - 1.675 0.044 - 0.047
    Demantoid (Andradite) 1.880 - 1.888
    Diamond 2.417
    Diopside 1.664 - 1.721 0.024 - 0.031
    Dioptase 1.645 - 1.720 0.053
    Dolomite 1.500 - 1.703 0.179 - 0.185
    Dumortierite 1.668 - 1.723 0.150 - 0.370
    Ekanite 1.590 - 1.596 0.001
    Emerald (Beryl) 1.575 - 1.602 0.004 - 0.009
    Emerald (synth. flux) 1.553 - 1.580 0.003 - 0.005
    Emerald (synth. hydro) 1.563 - 1.620 0.003 - 0.008
    Enstatite 1.650 - 1.680 0.010
    Epidote 1.715 - 1.797 0.015 - 0.049
    Euclase 1.650 - 1.677 0.019 - 0.025
    Fayalite (Olivine) 1.827 - 1.879 0.052
    Fluorite 1.432 - 1.434
    Friedelite 1.625 - 1.664
    Gahnite 1.790 - 1.820 (isometric)
    Gahnospinel 1.735 - 1.790
    Genthelvite 1.742 - 1.745
    Glass (man-made) 1.520 - 1.550
    Gold 0.470
    Goshenite (Beryl) 1.566 - 1.602 0.004 - 0.008
    Grossular (Garnet) 1.730 - 1.760
    Hackmanite 1.483 - 1.487
    Hambergite 1.550 - 1.630 0.072
    Hauyne 1.496 - 1.505
    Heliodor (Beryl) 1.566 - 1.579 0.005 - 0.009
    Hematite 2.880 - 3.220 0.280
    Hemimorphite 1.614 - 1.636 0.022
    Hessonite (Garnet) 1.742 - 1.748
    Hiddenite (Spodumene) 1.653 - 1.682 0.014 - 0.027
    Howlite 1.583 - 1.608 0.022
    Hydrogrossular (Garnet) 1.690 - 1.730
    Hypersthene 1.686 - 1.772 0.017
    Idocrase 1.655 - 1.761 0.003 - 0.018
    Iolite 1.533 - 1.596 0.005 - 0.018
    Ivory 1.535 - 1.555
    Jadeite 1.640 - 1.667 0.012 - 0.020
    Jasper (Quartz) 1.544 - 1.553
    Kornerupine 1.665 - 1.700 0.013 - 0.017
    Kunzite (Spodumene) 1.653 - 1.682 0.014 - 0.027
    Kyanite 1.710 - 1.735 0.017
    Labradorite (Feldspar) 1.560 - 1.572 0.012
    Lapis Lazuli 1.500
    Lazulite 1.604 - 1.662 0.031 - 0.036
    Leucite 1.504 - 1.510
    Magnesite 1.509 - 1.717 0.022
    Malachite 1.655 - 1.909 0.254
    Maw-Sit-Sit 1.520 - 1.680
    Microline (Feldspar) 1.514 - 1.539 0.008 - 0.010
    Moissanite 2.648 - 2.691 0.043
    Moldavite 1.460 - 1.540
    Moonstone (Feldspar) 1.518 - 1.526 0.005 - 0.008
    Morganite (Beryl) 1.572 - 1.600 0.008 - 0.009
    Natrolite 1.473 - 1.496 0.012
    Nephrite 1.600 - 1.640 0.027
    Obsidian 1.450 - 1.520
    Oligoclase (Feldspar) 1.542 - 1.549 0.007
    Onyx 1.535 - 1.539 0.004
    Opal 1.370 - 1.470
    Orthoclase (Feldspar) 1.518 - 1.539 0.005 - 0.008
    Painite 1.787 - 1.816 0.027 - 0.028
    Pearl 1.530 - 1.685 0.155
    Pectolite 1.595 - 1.645 0.036
    Periclase 1.736
    Peridot (Olivine) 1.650 - 1.681 0.033 - 0.038
    Petalite 1.502 - 1.520 0.012 - 0.014
    Phenakite 1.650 - 1.695 0.016
    Phosphophyllite 1.595 - 1.621 0.021 - 0.033
    Prasiolite (Quartz) 1.544 - 1.553 0.009
    Prehnite 1.611 - 1.665 0.021 - 0.033
    Proustite 2.792 - 3.088 0.296
    Purpurite 1.850 - 1.920 0.007
    Pyrope (Garnet) 1.730 - 1.766
    Quartz 1.544 - 1.553 0.009
    Rhodizite 1.694
    Rhodochrosite 1.578 - 1.840 0.201 - 0.220
    Rhodolite (Garnet) 1.745 - 1.760
    Rhodonite 1.711 - 1.752 0.011 - 0.014
    Ruby (Corundum) 1.762-1.770 0.008 - 0.009
    Rutile 2.620 - 2.900 0.287
    Sanidine (Feldspar) 1.518 - 1.534 0.005 - 0.008
    Sapphire (Corundum) 1.762-1.770 0.008 - 0.009
    Sapphirine 1.714 - 1.723 0.006
    Scapolite 1.536 - 1.596 0.015 - 0.026
    Scheelite 1.918 - 1.936 0.016
    Serpentine 1.490 - 1.575 0.014
    Shattuckite 1.752 - 1.815 0.063
    Siderite 1.633 - 1.873 0.240
    Sillimanite 1.654 - 1.683 0.020
    Silver 0.180
    Sinhalite 1.665 - 1.712 0.035 - 0.037
    Smithsonite 1.620 - 1.850 0.227
    Sodalite 1.483 - 1.487
    Spessartine (Garnet) 1.790 - 1.810
    Sphalerite 2.400
    Sphene 1.900 - 2.034 0.100 - 0.192
    Spinel 1.712 - 1.735 (isometric)
    Spinel (syn. flame fushion) 1.710 - 1.740 (isometric)
    Spodumene 1.653 - 1.682 0.014 - 0.027
    Staurolite 1.736 - 1.762 0.011 - 0.015
    Strontium Titanate 2.400
    Taaffeite 1.717 - 1.730 0.004 - 0.009
    Tantalite 2.260 - 2.430 0.160
    Tanzanite (Zoisite) 1.692 - 1.705 0.009
    Tektite 1.460 - 1.540
    Thomsonite 1.497 - 1.544 0.021
    Thulite (Zoisite) 1.692 - 1.705 0.006
    Tiger eye (Quartz) 1.544 - 1.553 0.009
    Topaz 1.609 - 1.643 0.008 - 0.011
    Tourmaline 1.620 and 1.640 (usually) 0.020
    Tremolite 1.560 - 1.643 0.017 - 0.027
    Tsavorite (Garnet) 1.560 - 1.643 (isometric)
    Tugtupite 1.494 - 1.504 0.006 - 0.008
    Turquoise 1.610 - 1.650 0.040
    Ulexite 1.496 - 1.519 0.023
    Uvarovite (Garnet) 1.740 - 1.870 (isometric)
    Vanadinite 2.350 - 2.416 0.066
    Variscite 1.560 - 1.594 0.031
    Vesuvianite 1.655 - 1.761 0.003 - 0.018
    Vivianite 1.569 - 1.675 0.040 - 0.059
    Water (at 20°C) 1.3328
    Willemite 1.690 - 1.723 0.028
    Wulfenite 2.280 - 2.405 0.122
    Zincite 2.013 - 2.029 0.016
    Zircon, High 1.970 - 2.025 0.000 - 0.008
    Zircon, Medium 1.840 - 1.970 0.008 - 0.043
    Zircon, Low 1.780 - 1.850 0.036 - 0.059
    Zoisite 1.685 - 1.725 0.004 - 0.008

    Sources

    • Gemmology 3rd edition (2005) - Peter Read
    • Gemology - C.S. Hurlbut and G.S.Switzer (1981) Gemology. New York, USA., Wiley, 1st ed., 243 pp.
    • Gems, Their Sources, Descriptions and Identification 4th edition - Robert Webster, Anderson
    • Gem Identification Made Easy 3th edition - Bonanno, Antoinette Matlins
    • Gem-A Foundation and Diploma notes
    • Refraction Anomalies in Tourmalines - R. Keith Mitchell, Journal of Gemmology Vol. 10, 194 (1967)
    • Better refractometer results with the Bright Line technique - Dr D.B. Hoover and C. Williams, Journal of Gemmology Vol. 30 No. 5/6, 287-297 (2007)
    • The Tourmaline Group (1985) - Richard Dietrich ISBN 0442218575