(_Rock-Crystal_, _Amethyst_, _Citrine_, _Cairngorm_, _Cat’s-Eye_, _Tiger’s-Eye_) Although the commonest and, in its natural form, the most easily recognizable of mineral substances, quartz nevertheless holds a not inconspicuous position among gem-stones, because, as amethyst (Plate XXVII, Fig. 7), it provides stones of the finest violet colour; moreover, the yellow quartz (Plate XXVII, Fig. 5) so ably vies with the true topaz that it is universally known to jewellers by the name of the latter species, and is too often confounded with it, and the lustrous, limpid rock-crystal even aspires to the local title of ‘diamond.’ For all purposes where a violet or yellow stone is required, quartz is admirably suited; it is hard and durable, and it has the merit, or possibly to some minds the drawback, of being moderate in price. Despite its comparative lack of ‘fire,’ rock-crystal might replace paste in rings and buckles with considerable advantage from the point of view of durability. The chatoyant quartz, especially in the form known as tiger’s-eye, will for beauty bear comparison with the true cat’s-eye, which is a variety of chrysoberyl. Except that cat’s-eye is cut _en cabochon_, quartz is step- or sometimes brilliant-cut. Ranking with corundum next to diamond as the simplest in composition of the gem-stones, quartz is the crystallized form of silica, oxide of silicon, corresponding to the formula SiO_{2}. When pure, it is entirely devoid of the faintest trace of colour and absolutely water-clear. Such stones are called rock-crystal, and it is easy to understand why in early days it was supposed to represent a form of petrified water. It is these brilliant, transparent stones that are, when small, known in many localities as ‘diamonds.’ Before the manufacture of glass was discovered and brought to perfection, rock-crystal was in considerable use for fashioning into cups, vases, and so forth. The beautiful tints characterizing quartz are due to the usual metallic oxides. To manganese is given the credit of the superb purple or violet colour of amethyst, which varies considerably in depth. Jewellers are inclined to distinguish the deep-coloured stones with the prefix ‘oriental,’ but the practice is to be deprecated, since it might lead to confusion with the true oriental amethyst, which is a purple sapphire, one of the rarest varieties of corundum. Quartz of a yellow hue is properly called citrine, but, as already stated, jewellers habitually prefer the name ‘topaz’ for it, and distinguish the true topaz by the prefix Brazilian—not a very happy term, since both the yellow topaz and the yellow quartz occur plentifully in Brazil. Sometimes the yellow quartz is termed occidental, Spanish, or false topaz. Stones with a brownish or smoky tinge of yellow are called cairngorm, or Scotch topaz. The colour of the yellow stones is doubtless due to a trace of ferric oxide. Stones of a smoky brown