11.04: Refractometer

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

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 n_D 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 n_D). 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

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

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

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

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.

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
1.527	1.527	1.527	1.527	1.527	1.527	1.527	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
1.544 ω	1.553 ε	1.544 ω	1.552 ε	1.544 ω	1.549 ε	1.544 ω	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
1.613	1.619	1.611 α	1.616	1.614	1.619	1.611 α	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.

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.

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.

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 n_b is closer to n_a, the gem is (+)

2. If n_b is closer to n_g, the gem is (-)

3. If n_b is halfway between n_a and n_g, 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

• n_w = omega, the constant RI of a uniaxial crystal
• n_e = epsilon, the variable RI of a uniaxial crystal

Biaxial crystals

• n_a = alpha, the lowest RI of a biaxial crystal
• n_b = beta, the intermediate RI of a biaxial crystal
• n_g = 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

Related Topics

• Refraction
• Brilliance
• Dispersion
• Interference
• Immersion cell

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