Diamond gemstone

by Manisha G

diamond-gemstoneChemical Composition : Carbon C

Crystal system: Isotropic or Cubic System

Habit : The octahedron and the more complex trisoctahedron and hexoctahedron (form in which the normal octahedral face is replaced by three faces and six triangular faces respectively), rhombic dodecahedron may also occur. Twinned octahedra, known as macles, are common. Whereas the tetrahedron (a form consisting of 4 triangular faces only) and the cube forms are of rarer occurrence.

Most diamond crystals show curved faces, often with grooved edges and all octahedra are marked with triangular growth markings called “trigons”. The surface of the crystal is often almost resinous, in appearance and the true lustre and colour is not seen until the stone is cut.

Varieties :

1)    Gem Variety : Highly transparent, colourless or with shades of  colour and largely free from flaws.

2) Industrial Varieties (the major part of the output) :

Boart (Bort): This term is used today for any quality, too poor for gem purposes. Much of this material is multicrystalline, which adds toughness to the supreme hardness of the stones. Boart is crushed to provide diamond powder used as an abrasive in many industries, including diamond cutting.

Carbonado : This is the more valuable of the industrial types of diamond. The material is generally opaque and grey or black, the structure is microcrystalline, almost cryptocrystalline, and is matted to such as an extend that cleavage cannot occur. Carbonado is used extensively in rock saws and drills.

Coated Diamonds : Which have a thin to very thick “skin” or “coat” made of multicrystalline diamond with some mineral or clay intrusions surrounding a single crystal of diamond. Some of the thin skinned diamonds are now-a-days cut to produce good quality gemstones.

Physical Properties :

Hardness : 10. Diamond is hardest of all natural substances. The hardness varies with the direction being at right angles to the octahedral (cleavage) face. Diamond powder cuts diamond because of this – a hard direction in the powder will cut across the softer direction of the diamond – and the powder in the cutting lap presents all possible directions to the stone being cut.

Cleavage : Occurs readily parallel to the octahedral faces giving four directions of possible cleavage. The stone can be cleaved in uny plane parallel to any of these directions.

S.G. : Specific Gravity, 3.52 is very constant for gem quality diamond. Boart and carbonado have variable and lower densities due to presence of impurities.

Lustre : Adamantine. This is the highest degree of lustre possible in transparent substances. In diamond it adds greatly to the brilliance and “life” of the gem. It is entirely a surface effect and must not be confused with the lively reflections from the inside of the stone. To assess the lustre, move the stone until an image of the light source is seen reflected in the table facet. If the stone is well cut and polished this image will be very bright and free from distortion. If a similar test is tried with any other colourless gem, it will be found that the image is rather less intense and probably distorted by a slight curvature of the facet.

Transparency : Fine diamond is a gem of quite remarkable transparency and light passing through it is practically undiminished in intensity. This gives it a limpid quality known as “water” and, although the term is little used today.

Refraction : This is a gem of the cubic system so the refraction is single and does not vary with direction. The refractive index is very high (2.418) and is constant for all gem varieties. It is this Factor which leads to the high lustre and dispersion of the stone. Individual stones examined between crossed nicols often show slight anomalous double refraction due to mechanical strain caused by the release from conditions of immense pressure under which the diamond has formed. This birefringence is patchy and does not cover the entire stone. It is often concentrated around an inclusions of some other mineral.

Disperison : Although the dispersion of diamond is very great, (0.044 between the B and G lines), it is actually low when the enormous refractive power is considered. Thus when we have a substance such as synthetic strontium titanate with a similar refractive index, we find that the dispersion is very much greater. We also find that colourless zircon (0.038) approaches the dispersion of diamond, while sphene and demantoid garnet actually surpass it, (Dispersion of Sphene is 0.051 and Demantoid Garnet is 0.057); although their refractive indices are well down the scale compared with that of diamond (R.l. of Diamond is 2.418, Sphene 1.885 – 1.990 to 1.915 – 2.050 and that of Demantoid Garnet is 1.89). In the two last named stones the effect is marked by the body colour of the stone and is less obvious to the eye.

Colour : Under this heading some details of methods of classification are given. Diamond is the most important of the gems handled by the jeweller and although years of experience may be needed to judge diamond with certainty, there are some points of guidance which may be given. Diamond in gem quality is essentially colourless, but minute quantities of impurities give rise to faint shades of colour which have a profound effect on price. It is now known that most diamonds contain traces of nitrogen. Traces of iron and other colouring elements have been found and may have some influence on colour. Most type I diamonds show an absorption line in the violet at 4155A. This is particularly strong in slightly yellow or brown cape stones. Most diamonds fluoresce to a greater or lesser extent under U.V. light and the colour is usually a bright milky blue, although greens, yellows, pinks, orange, red shades also occur. Some blue fluorescing stones will show a marked yellow phosphorescence when the U. V. is removed. These are the rarest type II B blue which contains Boron as an impurity. The range of Intensity of the effects is very great and some stones scarcely fluorescence at all while others appear to be inert. X-ray fluorescence produces similar colours that give practically no phosphorescence.

A parcel of diamonds, when examined under S. W. or L.W.U. V. light will demonstrate the above, and the effect can be photographed vividly on a colour film. There is considerable variation in the colour and intensity of the fluorescence from stone to stone and it has been suggested that photography of this kind would serve effectively to “fingerprint” a larger diamond set piece of jewellery. This need not be done in colour since an ordinary black and white print will show sufficifent variations of intensity to give an adequate means of instant identification.

All diamonds of gem quality are transparent to X-rays and only type II diamonds are transparent to S.W.U.V. light. These properties are used to distinguish diamonds from its simulants (which are opaque to X-rays) and between the two classes of diamond.

Colour should be judged in good daylight ideally in a north light (south light in the Southern Hemisphere) on a clear bright day. Direct sunlight is no good and neither is artificial light of any kind. lAn exception might be made in the case of the special “diamondlite” madel specially for grading diamonds. “Dialite”, a light source marketed by System Eickhorst, Hamburg, Germany is a good source of mixed daylight having 6,500°K and 5,500°Kelvin.

Late afternoon and evening light are also useless and tend to make a yellow stone appear much finer than it really is.

Leave a Comment

Previous post:

Next post: