From time immemorial man has loved to bedeck himself with ornaments of all kinds, and every¬thing in nature which appealed to him as beautiful or mysterious, or a combination of both, has been eagerly sought both for the gratification which such possession gives, and for its commercial value in exchange for other commodities. The term “precious stones ” is usually used to include all those gems which are valued for ornament and jewellery. Strictly speaking, however, the only precious stones are the diamond, ruby, sapphire and emerald. although the term is often extended to the opal, not¬withstanding its lack of hardness, and to pearl, which is an animal product.

In this chapter, therefore, we have taken advantage of the extension and included also one or two gems which are either noted for some peculiar quality, or for which there is a peculiar demand on the part of certain people.
Of all precious stones the diamond, although not the most valuable, is the most celebrated and the most useful. A transparent, translucent mineral of pure carbon, the diamond derives its peculiar brilliancv from the fact that the back planes reflect all the light which strikes them at an angle exceeding 24° 13′.
The diamond is the hardest substance known and is able to scratch all other minerals. This quality has greatly extended its sphere as a commercial commodity The engraver uses it for etching-points, and the glazier uses it to cut glass. Of course for these purposes only a tiny splinter is used. The geologist and engineer also use it at the tip of their drill for boring rocks. This hardness in the diamond gave rise among the ancients to many superstitions concerning it. Even Pliny tells us that ” when struck on the anvil it returns the blow in such force that hammer and anvil are shattered. … It is incapable of being burnt.” The anvil test has spoiled many a diamond, and the Florentine Academicians in 1692 burnt the stone in the focus of a large lens.