Skip to main content
Geosciences LibreTexts

16.02: Diamond

  • Page ID
    3271
  • Diamond
    Chemical composition C
    Crystal system Cubic
    Habit Octahedral, cubic
    Cleavage Perfect (octahedral)
    Fracture Conchoidal
    Hardness 10
    Optic nature Isotropic
    Refractive index 2.417
    Birefringence None
    Dispersion High, 0.044
    Specific gravity 3.52
    Lustre Adamantine
    Pleochroism None

    Figure \(\PageIndex{1}\): The Hope Diamond weighs 45.52 carats

    Diamond is a polymorph (many forms) of the element carbon. Graphite is another polymorph. These two minerals share the same chemistry -- pure carbon -- but have very different structures and properties. Diamond is hard, graphite is soft (the "lead" of a pencil). Diamond is an excellent electrical insulator, graphite is a good conductor of electricity. Diamond is the ultimate abrasive, graphite is a very good lubricant. Diamond is transparent, graphite is opaque. Diamond crystallizes in the isometric system and graphite crystallizes in the hexagonal system. Somewhat of a surprise is that, at surface temperatures and pressures, graphite is the stable form of carbon. In fact, all diamonds at or near the surface of Earth are currently undergoing a transformation into graphite. This reaction, fortunately, is extremely slow.

    Diamonds have long been valued for their supreme hardness and incomparable brilliance. Chemically, a diamond is pure carbon just like the graphite used in pencils. Diamond's hardness is the result of extremely strong covalent bonding between the carbon atoms. Although most people think of diamonds as colorless, they actually occur in almost every color. Diamonds were viewed as talismanic by the ancient Hindus in India, which is where diamonds were first discovered. The most powerful stones were thought to be naturally occurring octahedrons of exceptional clarity which exhibited fire. These stones would bring the owner power, wealth, everlasting youth and good fortune. It was believed that flawed or inclusive stones could have quite the opposite effect. During the 1st century AD, prominent Romans wore uncut diamonds set in rings also as talismans. For hundreds of years, it was believed diamonds had gender. As late as 1566, Francois Ruet described two diamonds as having offspring. The first diamond engagement ring was given to Mary of Burgundy by Maximillian in 1477, thus establishing the tradition.

    Diagnostics

    Color

    Diamonds are found in all colors, but diamonds most commonly occur in shades of yellow and brown. Colorless diamonds which are graded D-E-F are much rarer. The rarest colors are reds, blues, pinks, and greens of intense saturation.

    Dot test

    When a well proportioned brilliant is placed, table down, on a sheet of white paper with a black dot on it, the dot should not be visible through the pavilion of the stone. This is due to total internal reflection.
    Diamond simulants with a lower index of refraction will show the dot on multiple pavilion facets, in a ring pattern.
    Similarly, when a clear brilliant of good cut is placed over a colored piece of paper, the color will not shine through.

    Brilliants that are not well proportioned, as fish-eyes, may also show the ring of dots, so one should assess the diamond's cut first.

    An alternative way of performing this test on set stones with an open back, such as solitaires, is to enscribe the dot on the table and observe it from the pavilion. Be sure the dot can be easily cleaned after.

    Spectrum

    Figure \(\PageIndex{2}\): Spectrum of yellow diamond


    Figure \(\PageIndex{3}\): Spectrum of brown diamond

    Diamond Grading: The 4 C's

    The 4 C's, when referring to diamond value, are color, clarity, cutting proportions and carat weight. All four components are equally important in determining the final value of a diamond. The criteria for diamond grading, most respected internationally, was developed by the Gemological Institute of America (the G.I.A.), during the 1950s. The following terminology and systems described are those of the G.I.A.

    Color Grading

    Diamondcolorscale.jpg

    Figure \(\PageIndex{4}\)


    The color of a diamond refers to the relative amount of yellow, brown or gray body color that a stone possesses. The G.I.A. scale starts at "D" and goes through "Z", with "D" being void of any body color, and "Z" having a light yellow, brown or gray color.

    Clarity Grading

    Clarity grading of diamonds is defined by visual observation by a professional with the aid of a 10x loupe or the naked eye.

    The most used system is the GIA system of clarity grading.

    Table \(\PageIndex{1}\)

    Main grade Subgrade Short grade explanation
    Loupe clean FL Flawless Internally and externally flawless when examined with a loupe
    IF Internally Flawless Internally flawless
    VVS VVS 1 Very very slightly included
    (high end-close to IF)
    Very, very small inclusions may be visible when examined with a loupe but the inclusions are not on the table
    VVS 2 Very very slightly included
    (low end-close to VS 1)
    Very, very small inclusions may be visible when examined with a loupe
    The distinction between VVS 1 and VVS 2 is only made for stones above 0.5 ct
    VS VS 1 Very slightly included
    (high end-close to VVS 2)
    Very small inclusions are present that are hard to see with the loupe but the inclusions are not on the table
    VS 2 Very slightly included
    (low end-close to SI 1)
    Very small inclusions are present that are hard to see with the loupe
    SI SI 1 Slightly included
    (high end-close to VS 2)
    Small inclusions are easily observed with the loupe but the inclusions are not on the table
    SI 2 Slightly included
    (low end-close to I 1)
    Small inclusions are easily observed with the loupe
    I I 1 Imperfect 1 Inclusions are hard to observe with the naked eye, but easily with the loupe
    I 2 Imperfect 2 Inclusions are hard to observe with the naked eye but occur in multiple sites
    I 3 Imperfect 3 Inclusions are very easily observable with the naked eye and interfere with the brilliance of the stone

    Cut: Proportion Analysis

    Carat Weight

    Carat is a unit of internationally recognized metric weight and does not refer to size. There are 5 carats in a gram. The weight of a diamond is measured in carats. A carat is divided into 100 parts, each called a point. Each point weighs 2 milligrams.

    Durability

    Diamonds are the hardest of all gemstones in their ability to scratch, but they can be broken along their four planes of inherent cleavage. Historically, it has been a very foolish practice to test for identification by using a diamond phonograph needle to scratch a suspected diamond. Only a diamond can scratch a diamond and semi-destructive testing of this sort is not necessary, given modern gemological instrumentation such as thermal inertia testing devices. Diamonds and many other hard gemstones easily scratch glass, so this is also a useless test of identification. Fashioned or cut diamonds are vulnerable to chipping at the polished or unpolished girdle edge as well as at the bottom of the pavilion which is known as the culet. The culet is the smallest possible facet and may or not be present. Diamond cutters usually strive for maximum weight retention from the rough and sometimes omit the culet. Small culets are considered acceptable by diamond graders and serve as the 58th facet in what is known as the standard "round brilliant cut."

    Localities

    Diamond deposits are found worldwide, the most notable deposits being on the continents of Africa and Australia, India, and most recently Canada.

    Treatments

    Definition
    Diamond treatments refer to the processes done to alter the appearance of a diamond by enhancing one of the qualities of the stone, most commonly its clarity or color.

    Laser Drilling

    Figure \(\PageIndex{5}\): Laser Drill Hole

    This treatment involves pointing a high powered laser directly at a dark inclusion within a diamond and burning a tiny tunnel towards it. With luck, the inclusion will be altered enough by the heat of the laser to make it less noticeable. If that is not the case, a strong acid can be forced down the drill hole which will dissolve the inclusion and make it less obvious. This treatment is usually easily detected with 10x magnification. The drill hole breaks the surface of the diamond and leads in a straight line to the treated inclusion. Occasionally drill holes can be filled with glass (the same process as clarity enhancement) which makes them more difficult to detect with magnification. Laser drilling was first encountered in the 1960s.

    In 2000, a new type of laser drilling was developed in Israel. It is called Kiduah Meyuhad (KM). The laser focuses on an inclusion within a diamond, and causes a series of internal fractures that make the inclusion appear to "bleach out". This laser treatment looks more natural and leaves no drill holes. It resembles feather like inclusions in the diamond, which do not break the surface. Detection of diamonds that have been treated with KM can be very difficult and requires the examiner to have experience in recognizing the characteristics of these microscopic fractures. Some KM treated diamonds have not been detected by major gem labs when grading.

    Coating

    Diamonds have been coated to disguise a low color grade, or to produce "fancy colors". The stability of the treatment varies, depending on the method used. Coated stones need special care, as aggressive ultrasonic cleaning, repairs, or re-cutting may destroy the integrity of the coating and totally change the appearance of the stone.
    For more information, read this article from Gems & Gemology.

    Clarity Enhancement

    Figure \(\PageIndex{6}\): The "flash" in a clarity enhanced diamond

    Clarity enhancing a diamond involves replacing the air in a surface reaching crack or cleavage with a substance having a similar refractive index to the diamond. This changes the relief of the inclusion making it much more difficult to see. Clarity enhancement can have a dramatic effect on improving the look of a diamond.
    To the trained eye, clarity enhanced diamonds are usually detected easily with magnification. The early examples of this technique produced broad flashes of color, bright red, violet, and orange, in the areas of treatment. The early glass filling was not very stable and was often damaged or destroyed with ultrasonic cleaning, repairs or normal wear.
    The new generation of clarity enhancement does not produce the obvious flashes of color when viewed with magnification. Although, careful observation can reveal gas bubbles, flow patterns and partial crystallization of filler components. Still, a faint color flash can be observed, but this effect is similar to the colors seen as the result of natural "strain" which produce a rainbow-like pattern. So experience is necessary for positive detection. The flashes of color seen in a clarity enhanced diamond are one color at a time, whereas the colors produced by strain are spectral in appearance.

    These companies specialize in clarity enhancement:

    Irradiation to Produce Color Enhancement

    Figure \(\PageIndex{7}\): Unnatural blue/green color produced by irradiation

    The earliest experiments with diamond irradiation date back to 1904, when diamonds were exposed to radium salts, producing a greenish coloration. These early treated diamonds proved to be radioactive, thus posing health concerns to anyone handling them. Diamonds treated with radium salt are occasionally encountered in period pieces of jewelry and should be tested with a Geiger counter as the radioactivity may still linger.
    Diamonds can also be bombarded with neutrons from a cyclotron to cause a change in color. The penetration is shallow so only stones that have already been fashioned are treated in this way. Detection of this cyclotron irradiation is quite easy as it produces a color concentration known as the "umbrella effect" The development of nuclear reactors, allowed several diamonds to be treated at a time and the depth of penetration was much greater. This allowed rough diamonds to be treated. The original color produced by treatment was green, but the diamonds could be annealed to produce pink, red, yellow, blue and orange. But these colors are not always stable when subjected to high heat, so care must be taken during repair work, using a torch. It is very difficult to detect this treatment without very sophisticated laboratory equipment.

    HPHT-High Pressure High Temperature Treatment

    HPHT treatments evolved from early research General Electric conducted in the 1950s in the quest for synthetic diamonds. This process is currently used to remove color from certain diamond classes (Type IIa) or to create fancy colors in Type IIa diamonds in combination with other procedures. The treatment involves subjecting the diamond to heat and pressure similar to those occurring when the diamond initially crystallized. It is claimed that this process "repairs" the crystal lattice, and changes the color. A color change of Z to D has been reported.
    HPHT is used in a combination with other treatments to produce fancy colors such as blue, orange, red, green and yellow.
    For further information read:

    Synthetics

    Swedish and American researchers discovered how to synthesize diamonds in the 1950s. Currently, the two methods used to synthesize diamonds are High Pressure, High Temperature (HTHP) and Chemical Vapour Deposition (CVD).

    Diamond synthesis has entered a new phase of development with the expansion of vacuum science and our understanding of molecular physics. At the atomic level, diamond is not unlike pure carbon or graphite in that it can be "built from the ground up." Precipitating diamonds using various Chemical Vapor Deposition techniques has been a slow process up to now. Much of the testing is already done and private labs are now breaking ground at an accelerated pace. What would take 200 hours to precipitate, is now being done in 24 hours.

    Types of Synthetics
    There are two types of synthetics High Pressure High Temperature (HPHT) and Chemical Vapor Deposition (CVD).
    1. HPHT: this type of synthetic is produced by subjecting a piece of graphite to extreme pressure and heat resulting in a stone being produced that is up to one carat in size and the equivalent of SI clarity. The color of stones produced through this technique are generally yellow-orange and yellow-brown and very rarely nearly colorless.

    2. CVD: a combination of methane and hydrogen gases are subjected to an existing polished natural diamond, in an environment heated and below normal pressure resulting in the gaseous vapor condensing on the surface of the natural diamond to form a larger synthetic. Synthetics produced by CVD techniques range from nearly colorless to brown.

    Detection methods
    Physical characteristics: different crystal growth patterns and presence of a seed crystal; hourglass shaped grains; color zoning; lots of inclusions; multiple growth faces of HPHT synthetics leads to a cube-octahedral shaped as opposed to an octahedral shape of natural diamonds. CVD produced synthetics are tabular or block shaped; fluorescent; clouds of white particles located along a plane. Under a microscope: synthetics have lots of inclusions; inclusions located on the girdle; fractures; black spots can be seen in HPHT synthetics as the result of overheating that has lead to graphite fracturing.


    Tests:
    -Spectroscopic tests such as infrared and UV.
    -Cathode luminescence shows cross-like growth sectors in HPHT synthetics.

    -Strain patterns are visible under polarized light.
    -HPHT phosphorescence (afterglow) once ultraviolet lamps are turned off.
    -Fluorescence patterns differ to patterns in natural diamonds. Yellow-green fluorescence in a yellow stone indicates that it is a HPHT synthetic; whereas red fluorescence in a brown or colorless stone highlights that it is a CVD synthetic.

    -Magnetic tests find that synthetics can be moved by a powerful magnet whereas diamonds cannot.

    Disclosure requirements
    - A synthetic must always be disclosed. The use of the word ‘diamond’ can only be used in conjunction with the term synthetic; man-made or artificial.
    -Misleading descriptors of synthetics such as ‘cultured’ cannot be used.
    - Firms that deal in synthetics can only use terms as listed above in their name.
    -Gemological laboratories produce reports for synthetics with the synthetic nature of the diamond fully disclosed.

    Simulants

    Synthetic cubic zirconia, moissanite, yttrium aluminum garnet (YAG), gadolinium gallium garnet (GGG), strontium titanate, lithium tantalite, lithium niobate, synthetic rutile, leaded crystal (glass imitation).

    Sources