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DIAMOND

Diamond Crystal E Very frequently two rhombic dodecahedra or two hexakis-octahedra are twinned according to the same law on a face of the octahedron, that is, the two individuals have a face in common which occupies the position of an octahedral face, and about which they are symmetrical. These twin-growths also are much compressed in a direction perpendicular to the twin-plane, this is illustrated in Fig. H, which represents a lenticular, or heart-shaped, crystal with curved faces. In this crystal only six faces of each of the hexakis-octahedra are developed, and these form low six-sided pyramids with a common base parallel to the six-sided octahedral face shown on the figure. Fig. I, represents another kind of twin growth of rarer occurrence, in which the crystal has the form of a rhombic dodecahedron (Fig. E). Parallel to one or more of the possible faces of the octahedron, which if present would truncate the corners in which three edges meet, are very thin lamellae in twin positions to the main crystal. Large numbers of these twin-lamellae may be present, and give rise to striations on the faces of the crystal.

Striations due to the same cause may also be present on the faces of the hexakis-octahedron, where, as before, they are parallel to one or other of the octahedral faces.

All these twin-groupings are quite regular and conform to certain definite crystallographic laws. Other intergrowths of two or more diamond crystals may be met with, in which the grouping is irregular and accidental, and cannot be referenced to any general rule, the relative positions of individual crystals being determined by chance. In such inter-growths may be found small crystals growing singly on a larger one, or several crystals of more or less equal size may be united in an irregular group. Such groups are unsuitable for cutting as gems and are usually devoted to technical purposes, the same is true to a certain extent in the case of the twinned crystals above described. Irregular groupings of diamond crystals may, in a crystallographic sense, be referred to as bort, in the technical sense, However, the term bort includes all stones which, from some reason or another, are unfit for use as gems, and this term is even applied to simple crystals disfigured by some serious fault, such as imperfect transparency, bad color, etc. Bort occurs in a peculiar spherical form, being built up of a large number of small crystals radially arranged, so that the whole group takes the shape of a more or less perfect sphere. Numerous small points project from the surface of the sphere, these being the corners of the individual crystals forming the group. These spheres of bort are found in all diamond mines to the extent of from two to ten per cent of the total output. Not infrequently only the outer shell of the sphere has the radially fibrous character just described, the central portion being occupied by a large, regularly-formed single crystal, which is usually so loosely attached to the radially crystalline shell that it falls out when the latter is broken.

Massive diamond with a granular crystalline structure and a black color is known as carbonado or "carbonate". Since it is applied to technical purposes only, it may be regarded in this sense as bort. It is found almost exclusively in the State of Bahia in Brazil, its characters will be further described when the occurrence of diamonds at this locality is presented.

SIZE OF DIAMOND CRYSTALS.

The size of diamond crystals varies between somewhat wide limits. The smallest sometimes measure less than a millimeter in diameter, but still smaller specimens (diamond sand) occur in nature. Small stones, measuring not more than one-quarter or one-third of a millimeter along the edge may be separated from a parcel of Brazilian diamonds by sifting with a sieve of fine mesh, the majority of these are octahedra, while cubes and rhombic dodecahedra are but rarely present. The faces of these very small crystals have the same surface characters as those of the larger crystals. By carefully washing for diamonds on the Cape diamond-fields, it is possible to obtain many stones very much smaller than those which usually come into the market, some indeed weighing no more than ~ carat. In the method of washing formerly practised at the Cape and also in Brazil, a large number of the smallest diamonds were lost, their value not being sufficient to justify a special collection of them, the improved washing machinery now in use is, however, capable of saving all the stones however small.

Stones of microscopic dimensions have only recently been observed, previous statements of supposed occurrences, such, for example, as their presence in the xanthophyllite of Zlatoust in the Urals, being based on errors of determination. Microscopic diamonds have been observed in large numbers in the diamond-bearing rock of the Cape, and there is no reason to doubt that they are present in other diamantiferous deposits.

Smaller diamonds occur in larger number, larger stones are more limited in number, while very large specimens are so extremely rare and valuable that they are known by special and distinctive names, and in most cases form part of the crown jewels of various countries, these famous stones will be described further on in a special section.

The average size of diamonds found in different countries varies very considerably, formerly, when India and Brazil were the only localities at which diamonds were known to exist, stones exceeding twenty carats in weight were of great rarity. During the most productive period of the mines of Brazil, two or three years would elapse before a second stone of this size would be found, while very few stones exceeding one hundred carats in weight were ever found. The largest stone ever found in this locality, known as the "Star of the South", weighed in the rough 254.5 carats. The "Braganza," of the Portuguese crown, said to weigh 1680 carats, would rank as the largest diamond ever found in any locality were it indisputably a diamond, the probabilities are, however, that it is a fine piece of colorless topaz.

The chances of obtaining large diamonds in the Indian deposits were more favorable, a considerable number of diamonds exceeding one hundred carats in weight having been found there. Most of the large Indian diamonds are only known in their cut condition so that their original weight can only be estimated. Of large Indian diamonds, known in the rough condition in recent times, the "Regent" French crown jewels, is the heaviest, it weighed before cutting 410 carats, and produced a beautiful brilliant of 136 14/16 carats. Other large Indian stones will be described below in the section on famous diamonds, they are comparatively few in number. The heaviest of the large diamonds of ancient times is known as the "Great Mogul," which is said to have originally weighed 787.5 carats, there is no authentic information, however, either as to its weight or to its present whereabouts. The island of Borneo has produced one or two large stones, the largest reported diamond, weighing 367 carats, is, however, like the" Braganza," almost certainly not diamond, and probably nothing more valuable than a piece of rock-crystal.

Striations due to the same cause may also be present on the faces of the hexakis-octahedron, where, as before, they are parallel to one or other of the octahedral faces.

All these twin-groupings are quite regular and conform to certain definite crystallographic laws. Other intergrowth of two or more diamond crystals may be met with, in which the grouping is irregular and accidental, and cannot be referenced to any general rule, the relative positions of individual crystals being determined by chance. In such inter-growths may be found small crystals growing singly on a larger one, or several crystals of more or less equal size may be united in an irregular group. Such groups are unsuitable for cutting as gems and are usually devoted to technical purposes, the same is true to a certain extent in the case of the twinned crystals above described. Irregular groupings of diamond crystals may, in a crystallographic sense, be referred to as bort, in the technical sense, However, the term bort includes all stones which, from some reason or another, are unfit for use as gems, and this term is even applied to simple crystals disfigured by some serious fault, such as imperfect transparency, bad color, etc. Bort occurs in a peculiar spherical form, being built up of a large number of small crystals radially arranged, so that the whole group takes the shape of a more or less perfect sphere. Numerous small points project from the surface of the sphere, these being the corners of the individual crystals forming the group. These spheres of bort are found in all diamond mines to the extent of from two to ten per cent of the total output. Not infrequently only the outer shell of the sphere has the radially fibrous character just described, the central portion being occupied by a large, regularly-formed single crystal, which is usually so loosely attached to the radially crystalline shell that it falls out when the latter is broken.

Massive diamond with a granular crystalline structure and a black color is known as carbonado or "carbonate". Since it is applied to technical purposes only, it may be regarded in this sense as bort. It is found almost exclusively in the State of Bahia in Brazil, its characters will be further described when the occurrence of diamonds at this locality is presented.

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This document is in the public domain.

March, 2011