From The Gemology Project
Revision as of 15:04, 14 February 2007 by Tom Goodwin, G.G. (talk | contribs) (Added "internationally recognized")
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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
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 co-valent 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.


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.


Spectrum of yellow diamond.

Spectrum of brown diamond.


Diamonds are the hardest of all gemstones in to 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 pavillion 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."


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


Laser Drilling

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 1960's.

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 recognising the characteristics of these microscopic fractures. Some KM treated diamonds have not been detected by major gem labs when grading.


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

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.
Clarity enhanced diamonds are usually easily detected 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.

For further information, check the websites of the companies that specialize in clarity enhancement:

Irradiation to Produce Color Enhancement

Un-natural blue/green color produced by irradiation

The earliest experiments with diamond irradiation dates 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 1950's 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 occuring 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:


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


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

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 1950's. The following terminology and systems described are those of the G.I.A.

Color Grading

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

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.

External links