A careful study of the showcases of the most extensive jewellery establishment brings to light the fact that, despite the apparent profusion, the number of different species represented is restricted. Diamond, ruby, emerald, sapphire, pearl, opal, turquoise, topaz, amethyst are all that are ordinarily asked for. Yet, as later pages will show, there are many others worthy of consideration; two among them—peridot and tourmaline—are, indeed, slowly becoming known. For the first five of the stones mentioned above, the demand is relatively steady, and varies absolutely only with the purchasing power of the world; but a lesser known stone may suddenly spring into prominence owing to the caprice of fashion or the preference of some great lady or leader of fashion. Not many years ago, for instance, violet was the favourite colour for ladies’ dresses, and consequently amethysts were much worn to match, but with the change of fashion they speedily sank to their former obscurity. Another stone may perhaps figure at some royal wedding; for a brief while it becomes the vogue, and afterwards is seldom seen. Except that diamond, ruby, emerald, and sapphire, and, we should add, pearl, may indisputably be considered to occupy the first rank, it is impossible to form the gem-stones in any strict order. Every generation sees some change. The value of a stone is after all merely what it will fetch in the open market, and its artistic merits may be a matter of opinion. The familiar aphorism, _de gustibus non est disputandum_, is a warning not to enlarge upon this point. PART I—SECTION A THE CHARACTERS OF GEM-STONES CHAPTER II CRYSTALLINE FORM With the single exception of opal, the whole of the principal mineral species used in jewellery are distinguished from glass and similar substances by one fundamental difference: they are crystallized matter, and the atoms composing them are regularly arranged throughout the structure.