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 1/32 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.
CLEAVAGE OF DIAMOND.
When a diamond crystal is broken on an anvil by a blow from a hammer, or when it is subjected to sudden changes of temperature, it breaks into a number of fragments which are usually bounded by perfectly plane and bright surfaces.
These surfaces of separation, or cleavage, will be found to have always a definite direction in the crystal, being parallel to one or in ore faces of the octahedron. If a chisel be driven into an octahedron of diamond in a direction parallel to an octahedral face the crystal will be divided into two portions, and the smooth, bright surfaces of separation will be parallel to the same octahedral face.
By suitably varying the position of the chisel, the crystal may be divided in the same way into two other portions of which the surfaces of separation will be parallel to any other of the faces of the octahedron. It is not possible, however; to produce a cleavage in a cube of diamond which shall be parallel to the faces of the cube; in these directions there will be irregular fractured surfaces only. If the chisel is so placed that a corner of the cube is removed, a cleavage surface will be produced, this being as before parallel to a face of the octahedron, the faces of which also truncate the corners of the cube in the natural crystals.
The cleavage surfaces of all crystals of diamond, whatever be their outward form, are always parallel to the faces of the octahedron, and in no other direction can plane cleavage surfaces be obtained. Diamond thus possesses an octahedral cleavage only, which is perfect and obtained with the greatest ease, this gem may indeed be regarded as one of the most perfectly cleavable of minerals.
This perfect cleavage with plane even surfaces throughout can, however, be obtained only in simple crystals. When we are dealing with an intergrowth of two or more crystals twinned according to the spinel-law or irregularly grouped together, the cleavage surfaces will have different directions in each individual, and will not pass uninterruptedly from side to side of the stone as in simple crystals; it is thus impossible to divide such a stone by a single cleavage surface.
From every simple diamond crystal, no matter what may be its external crystalline form, there may be obtained by cleaving parallel to all the octahedral faces a cleavage fragment having the form of an octahedron. The great importance of this property of cleavage in connection with the faceting of diamonds has already been touched upon, and will be again referred to under the special description of the process of diamond cutting.
The property of perfect cleavage is not, however, a desirable one from every point of view, for it is often responsible for the appearance of incipient cracks in the stone, which, if they further develop, seriously diminish its value.
SPECIFIC GRAVITY OF DIAMOND.
The specific gravity of diamond as determined by various observers varies between
3.3 and 3.7 g/cm3. Reliable determinations made on pure stones free from enclosures have however, in every case yielded values not lower than 3.50 and not much higher than this the mean value may, therefore, be placed at 3.52. The following are values obtained in particular instances by careful observers using pure material:
3.524 Brazilian diamond
3.524 Colourless and yellow diamond from the Cape
3.517 Brazilian diamond
3.529 "Star of the South" from Brazil
3.50 Diamond from Burrandong New South Wales
3.492 Colourless diamond from Bonieo
The fall of the last value below 3.5, is due to the attachment of a few air bubbles to the stone during the weighing in water.
Diamonds: Large and Famous Properties Geology and Mining Diamond Cutting Diamond trade
Rafal Swiecki, geological engineer email contact
This document is in the public domain.