Being a crystallized substance and excessively hard the Diamond is usually found in the form of more or less perfect crystals. These have forms such as the cube, octahedron, etc., which belong to the isometric system, and it is in this system that the Diamond crystallizes. The crystals do not possess, however, the highest isometric symmetry, but belong to the class designated by Groth as hexakistetrahedral, being tetrahedral with inclined face hemihedrism. It is very common for the faces to be curved instead of flat and to show etching figures of various kinds. The crystals are often considerably distorted so as to produce pointed and rounded forms, and twin crystals are common. Although so excessively hard the edges of the crystals as found in the beds of streams are often rounded from the wear of the other pebbles, probably chiefly quartz. Only the wear of centuries could produce such a result, however, for, as is well known, it is only with its own dust that the Diamond can be abraded to any appreciable degree by any of the means now used for cutting it.
DIAMOND AND CORUNDUM.
FROM BAUER’S EDELSTEINKUNDE.
First row: Sapphire Crystal. Diamond in Matrix (Brazil). Cut Sapphire. Second row: Ruby Crystal. Cut Ruby. Third row: Diamond in Matrix (South Africa). Fourth row: Bort. Black Diamond, Carbonado (Brazil). Fifth row: Spinel Crystal, Rubicelle. Spinel Crystal, Balas-ruby.
One important property of crystallized Diamond is that of cleavage parallel to the faces of the octahedron. This cleavage is of much service in preparing the gem for cutting, as by taking advantage of it, broad, flat surfaces can be obtained without grinding. This property also distinguishes Diamond from quartz, for which its crystals as found in sands are sometimes mistaken. Quartz has no cleavage. The fracture of the two minerals is the same, however, being conchoidal.
The massive forms of the Diamond known as bort and carbonado possess little or no cleavage, thus increasing their value as abrasives and for setting in drills, saws, etc. The true bort occurs as rounded forms made up of a confused aggregate of crystals and is harder than ordinary Diamond. Fragments of crystals of no value as gems or any crude Diamond dust are also known as bort in trade. Carbonado is a name given to black Diamond which has more or less crystalline structure. This graduates into the crystallized mineral. Either of these is more valuable than the crystallized Diamond for industrial purposes, although of no value as gems.
As already noted, Diamond occurs of various colors, about half the stones found being tinged to some degree. If the color is but slight, the stone is considered less valuable than if perfectly colorless, but a Diamond of pronounced color is the most valuable gem known.
Among colors of Diamonds, blue is the rarest. The largest and most valuable colored Diamond known is the Hope Blue, weighing 44½ carats. This is valued at about one hundred thousand dollars. It has a brilliant deep blue color and is without a flaw. A deep blue Diamond weighing 67-2/16 carats was long worn in the French crown, but it was stolen in 1792 and has never been recovered. Red Diamonds vary in hue from ruby red to rose, the latter being the most common. No large red Diamonds are known, the largest being one of 32 carats in Vienna. Another famous one is that in the Russian treasury, for which Paul I paid one hundred thousand roubles. It is of a ruby color. The finest green Diamond known is the “Dresden Green” preserved in the Green Vaults of Saxony. It was purchased by August the Strong in 1743 for sixty thousand dollars. It is apple green in color and weighs 40 carats. Diamonds of yellow color are comparatively common, many of the Cape Diamonds being lowered in value by possessing a yellow tinge. It is said that this injurious yellow tinge can be overcome by dipping the stone several times in a solution of potassium permanganate, the violet color of the latter neutralizing the yellow of the Diamond. The yellow tinge usually also disappears in artificial light. Of large Diamonds possessing a yellow color the Florentine and the Tiffany are the best known. The color of colored Diamonds is generally permanent, but that of some is said to fade on exposure to light. It can also be destroyed or changed by heat.
The luster of the Diamond is a peculiar one, and such as is possessed by few other minerals. In reference to its occurrence in the Diamond it is known as the adamantine luster. It combines the peculiarity of an oily luster with that of glass and that of a metal. It is doubtless due to the high refractive power of the mineral, which causes more than the ordinary number of rays of light to come to the eye. In the impure forms of Diamond the greasy or oily luster becomes more pronounced. Once the eye becomes accustomed to the peculiar luster of Diamond the stone may easily be distinguished by it from glass or minerals with a vitreous luster, such as quartz. Certain other minerals, however, such as cerussite, zircon, and to some extent sphene, exhibit the adamantine luster. In the glass known as strass, used to make imitation Diamonds, the adamantine luster is well imitated.
Diamond is usually transparent, but it may be translucent and even opaque, especially the black varieties. Even otherwise transparent Diamond often contains inclusions which cloud and interrupt its clearness. These constitute the “flaws” which so often injure the value of a Diamond and prevent it from being of the “first water.” These inclusions may be simply small cavities, sometimes so numerous as to make the stone nearly black, or they may be particles of other minerals, such as chlorite, hematite or carbonaceous matter. If the latter, the flaws can sometimes be burned out by careful heating.