The Properties and Determination of Diamonds

See also > Carat >Cut >Clarity > Colour > Fluorescence

The name “diamond” is a corruption of the Greek word “adamas” meaning invincible.

 Composition

Diamonds are composed of almost pure carbon.

 Crystallography

Diamonds are the isometric (cubic) form of crystalline carbon as opposed to the hexagonal form, graphite. Diamonds are the high-pressure polymorph of carbon and is meta stable at room temperature.

Diamonds, being the high-pressure form of carbon, exhibit close packing of the carbon atoms (lattice spacing of 1.54 ) such that each is surrounded by four others in a tetrahedral array yielding isometric forms of high symmetry. Diamonds display extreme covalence of bonding between atoms, the whole crystal being in effect on molecule. Any breakdown of the structure involves the breaking of strong covalent bonds and this gives diamonds both there chemical stability and extreme hardness.

Physical Properties of Diamonds

One of the most important consequences of the strong bonding of the diamond crystal lattice is extreme hardness. Diamonds are by far the hardest of all natural substances and are assigned a hardness of ten on the Moh scale.

The Moh scale is non-linear and relative. The hardness number “ten” only implies that diamonds are harder than corundum which is assigned the number “nine”.

 The hardness of diamonds vary with the direction in the crystal, the hardest direction being normal to the octahedral (cleavage ) face. This variation in hardness allows a diamond to be cut with diamond powder, because a hard direction will cut a softer direction and the powder in the cutting lap presents all possible directions to the diamond being cut. The wear rate in the soft direction of the diamond is about twice that of the hard direction of the diamond.

As all crystals of the same substance should have the same hardness, the term “soft” erroneously used to describe certain diamonds, refers to the lack of cohesiveness of a crystal aggregate. When diamonds of some regions are called “hard” reference is being made to the longer time taken in cutting and polishing the diamonds; this is a function of the number of knots (cross grain due to twinning) rather than a true hardness. Diamonds from whatever locality are about the same hardness.

In spite of extreme hardness, diamonds are a comparatively brittle substance and many valuable diamonds have been destroyed in consequence of an erroneous belief, that a genuine diamond, when placed on an anvil, can not be broken by a blow with a hammer. The real test of the hardness of a diamond is that it will scratch any other mineral, and can not in turn, be scratched by any other.

Cleavage occurs readily parallel to the octahedral faces, giving four directions of possible cleavage. The diamond can be cleaved in any plane to these directions. Diamonds have a conchoidal fracture.

 Specific gravidity varies from 3.516 to 3.525 for crystals,3.499 to 3.503 for bort. Owing to the purity of diamond crystals, it  is unlikely that the specific gravity of a diamond will deviate much from a value of 3.52.

Diamonds have a high thermal  conductivity (one third that of silver), a low coefficient of expansion, and a low coefficient of friction.

Diamonds are dominantly Colourless (white) and pale yellow ("off colour" or "capes"), though blue ,brown, red, orange, green, pink and deep-yellow diamonds are found. Deep shades of colour ("fancy diamonds") are rare in diamonds. Except for the pink diamonds whose colour is attributable to traces of manganese, the colour of diamonds is widely thought to be due to lattice defects rather than to trace elements. However, this is doubted by some authorities who attribute the yellow coloration in diamonds to minute amounts of nitrogen  ( as little as 1 ppm) replacing carbon in the lattice.

 Diamonds have such a brilliant lustre that they lend there name to the term "adamantine" This is entirely a surface effect due to the ability of the diamonds to take a polish and reflect light. In nature the surface of diamond crystals is usually resinous or greasy in appearance, or may be frosted due to innumerable edges on the crystal faces. The true lustre is not seen until the diamond is cut.

The refractive index is extremely high (2.418 for sodium light, the highest known for a Colourless mineral), is constant for all gem varieties, and is responsible for the luminous sparkle. Diamonds being isometric, are singly refracting, though slight anomalous double refraction has been observed in some diamonds due to mechanical strain - in these circumstances the birefringence is patchy and does not cover the entire diamond, often being concentrated around an inclusion of some other mineral.

Dispersion ("fire") is the breaking up of white light into the component colours of the spectrum. The effect is caused by differential refraction of different wavelengths (colours) of light The measure of dispersion generally used is the difference between the indices of refraction of the red and blue lines. Diamonds have a refractive index for blue light of 2.451 and red light of 2.407, and hence the high dispersion of 0.044. This high dispersion is responsible for the brilliant flashes of colour from well-cut diamonds.

Fluorescence of diamonds occurs both in long wave and in short wave ultraviolet light. The colour produced is generally a bright milky blue, but yellow and green fluorescence also occur. The range of intensity is very great, and some diamonds hardly fluoresce at all. It is not uncommon for the fluorescent glow in a diamond to be localized in bands or zones with some reference to crystal form. Variations in intensity of fluorescence may be used to "fingerprint" a piece of jewellery composed of many set diamonds, as the pattern obtained is unique. Many diamonds display an afterglow (phosphorescence) following exposure to ultraviolet light or sunlight. X-rays produce a similar fluorescence to that from ultraviolet light, as well as yellow phosphorescence.

Inclusions and Birthmarks

Cleavage cracks and feathery marks often mar the transparency of diamonds, and these may appear as black spots when seen at an angle such that light is totally reflected at their surfaces.

Inclusions are common in diamonds and occur as small crystals enclosed by the diamond lattice. The most common inclusions are magnesian olivine, haematite, magnetite, pyrope garnet, enstatite, zircon, diopside, rutile, pyrrhotite, and liquid carbon dioxide. The pyrope garnet inclusions are unusually rich in chromium.

Diamond itself is occasionally found as an inclusion in diamonds, but graphite is rarely found. Quartz has been found included only in Brazilian diamonds

Valuation

If imperfections in the diamond are undetectable under a lens magnifying ten diameters the diamond is said to be eye (or loupe) clean, i.e. flawless.

Refer to the following section for clarity grades: Diamonds>Diamond Grading>Clarity

Any inclusion which is visible to the naked eye is sufficient to detract markedly from the value of the diamond.

Absolutely Colourless diamonds are very highly prized. Fancy colours in diamonds are rare being generally faint shades of pink, blue, or green. Any diamond approaching a deep colour is rare enough to command a high price. 

Apart from colour and freedom from flaws, the size of a diamond has a dramatic influence on its value. The price of diamonds increase very rapidly with increase in size. It is rare for a diamond to exceed 5 carats in weight and large diamonds are often difficult to sell as the market for such diamonds is limited.

One of the most important factors influencing the value of a gem diamond is the quality and style of the cutting. Brilliant –cut diamonds tend to be the most highly valued, and diamonds improperly cut lose a great deal of value.

The cut of a diamond is expressed as a deviation from a standard adopted by a grading laboratory. Care should be taken of the loose use of the term “Ideal Cut” as there is no universally accepted standard. There is some overlap in the dimensional criteria used by the recognized laboratories  to establish a grade of “very good”.