Usually the Diamond is colorless or white, although shades of yellow are also common. It is also known in shades of red, green and blue and in brown and black. The two latter are rarely transparent and grade into the varieties known as bort and carbonado, which have no value as gems but are highly important for industrial purposes.
In composition the Diamond is pure carbon, thus not differing chemically from graphite or such forms of carbon as lamp-black, bone-black, etc. It is crystallized, but this can be said of graphite as well. Why carbon should assume the form of Diamond in one case and graphite in another, as well as being amorphous in other occurrences, is not known. Such behavior of a substance is known as dimorphism, and numerous illustrations of it are to be found in Nature.
Being pure carbon, Diamond can be burned in the air. The finely divided dust can be burned in the ordinary blow-pipe flame, and for stones of ordinary size a temperature of about 900° C is sufficient. The possibility of consuming the Diamond by heat is said first to have been suggested by Sir Isaac Newton, who reasoned from the high refractive index of the stone that it was “an unctuous substance coagulated,” and hence probably combustible. Following this suggestion two Italians, Averani and Targioni, succeeded in 1695 in burning some Diamonds in a furnace, and since then the experiment has been repeated many times. The Diamond does not fuse in burning, but after becoming heated to redness gradually grows smaller, emitting sparks, till it entirely disappears. It leaves no ash except in the case of the impure form known as carbonado. The gas given off has been collected and analyzed and found to be carbon dioxide just as would result from the combustion of other forms of carbon. If protected from the air or free oxygen, the Diamond can be exposed to high heat without change.
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.