DIAMOND
The most precious of gemstones, has been known from ancient times in Indian and other eastern countries, known as Vajra = hirak (Sanskrit); both the terms are used in the different Indian vernaculars. It is, commercially, also the most important gemstone, because it is the hardest, the most imperishable and the most brilliant of minerals. Attempts at manufacturing diamonds never proved a success and are only of scientific interest. It has the simplest composition. It is crystallized carbon, another form of which is graphite. Diamond has maintained its eminence as the most valuable of gemstones, but it is also becoming very valuable in industry which uses all the by product and non-precious diamonds.
The word diamond has been derived from the Greek and Latin terms, ‘Adamas’ (16 A.D.) whose original meaning was invincible. By usage the following changes took place as’ adamant’,’ diamant’ and’ diamond‘. Pliny (100 A.D.) speaks of the rarity of the stones. The Romans knew of it when it was introduced from India.
The discovery of diamonds in India predates the birth of Christ by several millennia. Kautilya’s Arthasastra, a work of third century B.C. refers to six kinds of diamonds from as many mines. It was about the fourteenth century that the cutting.processes were introduced from India to Europe, and the diamond did not come to its eminence till the fifteenth century. Till the middle ages, ruby, peridote and pearl were considered more precious. Indian lapidaries were the first to realize that diamond could be cut with its own powder, and that the removal of the skin in a natural stone made a great change in the stone.
In the fifteenth century, it became very popular and during the next two centuries, India had a flourishing diamond trade. Alas! the discovery of the rich Brazilian fields in 1725 dealt a death blow to the diamond fields of India. In the latter half of the nineteenth century, the discovery of the rich African fields proved also a disaster to the Brazilian fields.
Nevertheless, India still holds the reputation for having produced some of the finest and largest diamonds. South Africa now produces 95% of the world’s diamonds, whereas the Indian production is almost negligible. The world’s aggregate diamond production till about 1937 has been estimated at about 279 million carats or roughly about 61-5 tons. Now the world’s average annual production has exceeded 10 million carats. The production was first from gravels, then from pipe mines, which have been in turn superseded by the alluvial deposits as the chief producers.
II. Physical properties, etc.
Diamond crystallizes in the cubic system, the common forms being octahedron, rhombic dodecahedron, tetra-kis-hexahedron and spinel twins. The crystals are usually small, but larger crystals may also occasionally be found. The Indian stones are usually octahedrons, whereas the Brazilian stones are dodecahedrons. The Panna stones are hexakis-octahedron, tetrakis-hexahedron and hexakis-tetrahedron. The most common form is the octahedron, but the edges are not sharp and tend to curvatures near the corners, giving a rounded outline to the whole. Corners may not be present in some. In dodecahedrons the edges and corners are much sharper than in octahedron. The cube faces may be present as replacement forms in better diamonds. Irregular crystals are more numerous than those of good shape.
In alluvial stones, the original shape is not present due to wear and tear. The mine diamonds are often beautiful with triangular depressions or ‘trigons’ whose angles point to the octahedral edges. This enables the determination of orientation in broken crystals. The trigons are ‘etched figures’. In dodecahedrons, striations parallel to the longer diagonals of the rhombs are seen.
Twinning.—Octahedral and dodecahedral twins are common. The spinel twin is also called the ‘macle’. macles are rounded like simple crystals. Illusory twinning, ‘trigons’, are depressions. Elevations rising from a triangular base on the octahedral faces are also oriented conformably with the face. These elevations tend to arrange themselves symmetrically about the coigns of the crystal., They are nearly colourless and lose a great deal of weight in cutting.
Specific gravity.—The specific gravity of diamond is 3.5-3.6 Pure diamond has 3.53, the black variety 3.45
3.52 and bort 3.6. Hardness is 10 on the Moh’s scale of hardness.
It is the hardest mineral known, but it does not mean that it is twice as hard as apatite. In a scale representing relative hardness, the value for diamond would be 100 rather than 10. Diamonds from various areas are found to have varying hardness. The diamonds from Borneo and Australia are said to be harder than those from Kimberley. Perhaps some impurity might contribute to increased hardness. Hardness must not be confused with toughness, as diamonds are brittle and easily chipped. It has.also been found that hardness in diamonds varies with the different crystal faces and also with the direction of the faces. This fact is known to diamond cutters, as also the following: (a) faces parallel to the cube are easy to polish, as they are parallel to two crystal axes; (fe) rhombic dodecahedral faces are parallel to one crystal axis, and hence less easy to polish them than the faces parallel to the cube; and (c) the octahedral faces being equally inclined to three crystal axes and not being parallel to any crystal axis are the most difficult to polish. The change in hardness from one face to another or from one direction to another is not abrupt but gradual.
Cleavage is parallel to the octahedral face (111) and the mineral breaks along the four perfect cleavages parallel to the face of the octahedron. Dodecahedral cleavage is imperfect. Fracture is irregular and may vary also from conchoidal to splintery. Colour may vary from yellow, brown, grey and black. Each of them may have several grades. The bort belongs to the grey species and occurs in round and shapeless lumps and fragments. It has crystalline structure. The following are the varieties of diamond: (1) diamond proper, (2) bort or bortz, (3) ballas, and (4) carbonado.
Optical properties.—The crystals may be clear and transparent with a brilliant lustre on the surface, but may be also translucent and opaque. The transparent stones are valued as gems. The lustrous adamantine, but an uncut diamond may be dull or greasy. Carbonado may have a dull lustre. The full properties diamond, i.e. brilliancy, fire and play of .colours, for which the diamond is highly valued as a gem, are brought out by cutting and polishing.
Diamonds cut for jewellery are known as ‘brilliants’. These valuable qualities of diamond are due to its optical characters, e.g. refractive index 2-402 for red and 2.465 for violet and their difference 0-063 the dispersion. It is singly refractive or isotropic, but anomalous double refraction may be noted due to internal strains. Double refraction may also be noted surrounding enclosures or cracks. In respect of play of colours pris-matic) the Indian stone rank highest, followed by Brazilian and the Capa diamonds.
Inclusions of graphite are most common. Other minerals found as inclusions are ilmenite, chromite, magnetite and hematite. Garnet also occurs as rounded grains or frequent inclusions. It may enclose or be enclosed by diamond. Bubbles of liquid CO2 are also found. Diamond also occurs as an inclusion in diamond. In composition it is pure carbon. Diamond, graphite and charcoal are allotriomorphic forms of the same element carbon. Spontaneous explosion of diamonds has been reported due perhaps to accumulated strains. Phosphorescence is also common. It burns in air at 850°C.
It is insoluble in acids and alkalies. It becomes positively electrified by friction and is a non-conductor of electricity. When exposed to ultraviolet rays an electric current is produced through some diamonds. After prolonged exposure to radium a colourless diamond becomes green. Bort is a good conductor. It is also a good conductor of heat. It feels colder to touch than glass and imitation stones and has a greasy feel.
Fluorescence.—Under ultraviolet rays, some colourless diamonds show a cornflower blue glow and while greenish diamonds, show a greenish yellow glow. Diamond is much more transparent to X-rays than other minerals and much more so than glass imitations. The luminescence has been ascribed to the inclusions of some unknown hydro-carbon minerals. Dake arrived at the following conclusions: (1) non-fluorescent diamonds carried the least amount of metallic inclusions and may be considered the purest form of diamond, (2) the bort which fluoresced a strong blue contained the greatest amount of metallic elements, and (3) diamonds which fluoresced in different colours contain different proportions of various metallic elements.
The metallic elements found include chromium, iron, calcium, aluminium and manganese. The more common metallic element was chromium. In recent years, the crystal structure and optical characters of diamond have been studied intensively by Sir C. V.Raman and his co-workers at the Indian Institute of Science, Bangalore.
According to spectroscopic studies of his co-workers, it has been found that luminescence is not due to extraneous impurities, but is a property of diamond itself. This indication is further strengthened by the definite correlation which is found between luminescence and other properties, which differentiate the allotriomorphic modifications of diamond.
Fractured edges of cleavage plates of diamond luminesce far more strongly than the polished faces of interior of the crystal. So the inference is that luminescence is excited by purely physical cause, namely a disturbance of the regularity of the crystal structure. The variations in intensity of the luminescence may be explained as due to variation in the number and extent of such irregularities within the diamond.
It is also evident that an intimate interpenetration of positive and negative tetrahedra in diamonds of lower symmetry would constitute a disturbance in the regularity of crystal structure.
(*) ‘It has been found unnecessary to postulate the presence of extraneous impurities in the diamond, and if such impurities were the cause of luminescence, it would be difficult to understand why diamonds having the higher type of symmetry do not exhibit luminescence to anything like the same extent.
Size of diamonds.—The average size of diamonds obtained in different countries varies considerably. When India and Brazil were the only countries producing diamond, stones of more than 20 carats in weight were a rarity, and
(*) Raman, Sir C. V., and others. The structure and properties of diamonds, Current Science, Vol. XII, No. I, pp. 33-42 (Supplement), 1943
even such stones were found only once in two or three years. Stones exceeding 100 carats in weight were seldom found. The ‘Star of the South’ weighed in the rough 254-5 carats, though considered a large stone. There were greater chances of finding larger diamonds in the Indian deposits and a number of stones exceeding 100 carats in weight were obtained.
As most of the stones are known only in the cut condition, their original weight could only be estimated. Of the stones known in their rough condition ‘the Regent’ in the French Crown jewels is the heaviest; its weight before cutting was 410 carats and when cut yielded a beautiful brilliant of 136.875 carats. The ‘Great Moghul’ is supposed to have weighed originally 767.5 carats.
The South African fields yielded stones of up to 150 carats in weight more frequently, and some were of several hundred carats. The largest stone discovered was at the Cape in 1893, which weighed 971.75 carats. On 13th August, 1938, in the Santo Antonio river a pure blue-white diamond weighing 726-6 carats was obtained and named the Vargas diamond in honour of the President of Brazil. It was cut in the U.S.A. and divided into twenty-three stones.
On our present views, the latter fact finds a natural explanation in the circumstance that such interpenetration does not exist in the octahedral variety of diamond., The chemical or impurity theory is also discredited by the observation that strongly blue luminescent diamonds are often of highest quality in respect of transparency and freedom from colour.’