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It will be found, however, that our present conception of matter will not conform to a simple strict definition of a mineral. For instance, it would be difficult to express the composition of tourmaline by a fixed chemical formula, and there are minerals having similar chemical constituents which almost merge into each other and are yet different in appearance and name. Others contain traces of elements which affect their appearance or stability, yet these elements do not appear in any of their generally accepted formulae. What we have stated is a general rule only. Further, there are some materials used in jewelry which do not come within any definition of stones, such as pearls, coral, amber, and ivory, since these are of organic origin.

A generally accepted, although somewhat arbitrary, division exists between what are called “precious” and “semi-precious” stones. The latter term is misleading, but it is still used for want of a better description of those jewel stones other than diamond, ruby, sapphire, and emerald. These four stones are usually considered as being precious, although this term is not based solely on commercial value. There are numerous other stones which are both rare and beautiful, and at the same time reaching high commercial values. Such are, for instance, black opal and alexandrite. Jade necklaces have reached the price of $70,000 (£25,000) in America, while poor sapphires, a so-called precious stone, may cost less than a dollar a carat.

All such other stones are sufficiently attractive for ornamental purposes when they are suitably cut and polished. Many have depth of intrinsic color and are sufficiently hard to withstand ordinary wear, properties which are essential in gem stones. But commercial values vary greatly with fashion, and these stones are more subject to such current influences than are the precious varieties. The term semi-precious is therefore misleading; it does not mean half precious, or half real, and it would be as well if another term came into general use in its place. Among the many beautiful stones of this nature are zircon, chrysoberyl, topaz, spinel, jade, tourmaline, amethyst, and kunzite, all real stones which are seen in mounted jewelry but which, in many instances, are found in too abundant quantities to make them rare and therefore of high value. The essential qualities which make a stone “precious” are generally considered to be beauty, rarity, and durability.