After all, the most extraordinary property of the diamond, as pure carbon, is its weight. It is fifty times the weight of refined charcoal. How was the element of carbon condensed and inspissated thus? Was it by the action of LIGHT? This is the vegetable analogy; as we see in the case of all growing plants: did it hold here?
If we were to inquire how oriental sapphires, including the ruby, the blue sapphire, the emerald, and the amethyst, are formed from CLAY, that clay which exists in the granite rocks, the difficulty would be nearly as great. We do not know at all.
But there is some satisfaction in having undoubted proof of the nature of their “base.” And the obvious evidence upon this point is very simple, and prior to that of chemical analysis. My own attention was first drawn to it many years ago, in a casual remark made by a friend. We were handling some crystals of the white sapphire, a stone of little value. “These look very like glass,” said I. “Yes, but you may always tell them from glass by their coldness. Touch one with your tongue.” Then followed the inquiry. “And why is it so much colder than glass is?” “Because the ‘base’ of the sapphire is CLAY, and clay is a very cold substance.”
Let us now pass on to consider the question of COLOUR. In the existing varieties of gems, we have all the colours of the spectrum perpetuated and vivid.
What is the source of these colours? And how are they blended with the solid crystal?
There seems to be no reason for doubting that the immediate source or cause of colour is the presence of some oxidized metal. All the colours which Nature has impressed appear, as far as we can trace them, to be due to such a presence. Opaline tints, and those of the “cat’s eye,” are an exception; being the perceptible result of a peculiar texture and configuration; so also are the iridescent hues on a soap-bubble, which are probably caused by polarized light. But it is a metal which makes the bark of certain trees and shrubs to glisten; aided, in the case of the birch and wheat-stubble, by particles of silex. It is a metal, absorbed from juices of the soil, which gives their tints to flowers, and their deep tinge to fruits. It is a metal which dyes the plumes of the king-fisher; and the gleaming scales of the dragon-fly and diamond-beetle owe their brilliancy to metallic lustre. Why should this all-pervading law vary in the crystals?
We know that metals proper affect crystalline forms, assuming them spontaneously; and we know that the gems have come in contact with metals. Iron, whose rust is of a reddish hue, enters into many of them; and in the red stones it is said to be abundant.
Probably, the finer permanent tints are due to gold, in infinitesimal proportions. Professor Faraday has said that the ruby-tinted glass called “Bohemian” derives its colour from gold in a pure form, finely attenuated, and not from any chemical combination of that substance.
A white diamond has far more lustre than one which is yellow or violet-tinted. But in all the sapphires, the deeper the hue, the more brilliant and valuable the stone. Hence the technical term for the colour of a diamond is its water. In all these gems the hues (whatever their origin) are homogeneously united with the crystal; and the modus operandi of Nature is a profound secret.
When we descend in the mineralogical scale, and come to such stones as agates and jaspers, the process which has been followed seems quite capable of being traced—for its antiquity is not so great. The colours which we now find in pebbles more or less opaque, though occasionally lively, exist under very different conditions from those by which they reside in the crystalline gems. It is necessary, however, to consider first, the origin and nature of pebbles as distinguished from casual fragments of stone.