Magnesium is a common element and is likely to be present wherever lime is being deposited, so dolomite crystals are common, and much of the limestone is dolomitic. It may be found in almost any limestone section of the country. Some of the finest crystals of dolomite however come from Roxbury, Vt., Smithfield, R. I., Hoboken, N. J., Lockport, Rochester, and Niagara Falls, N. Y., etc. Silicon, Silica and the Silicates Silicon is one of the non-metallic elements, and does not occur as such in Nature. When isolated it is either a dark-brown powder, or steel-gray crystals. However silicon is next to oxygen in its importance in making the crust of the earth. Forty-seven per cent of the surface rocks are composed of oxygen, and 28% of silicon, the latter appearing in a host of minerals. The oxide of silicon is termed silica (SiO₂), its crystal form being quartz, the commonest of all minerals. In non-crystalline form silica is also widely distributed, as chalcedony and opal, even appearing in the tissues of animals and plants, as in the feathers of birds, the shells of certain Protozoa (Radiolaria), the spicules of sponges; and in plants, as the shells of diatoms, and in the stalks of grasses, especially cereals and bamboo. Silica in the form of sand is widely used in making glass, porcelain, china, etc., and in the various cements. Then there are a considerable number of acids of silicon, which do not occur in Nature, but their salts do, and make a host of minerals, which are known as the silicates, such as mica, feldspar, hornblende, etc. Either as quartz, or as silicates, silicon is represented in most all the igneous and metamorphic rocks and in many of the sedimentary rocks. Quartz SiO₂ Pl. 30 Occurs as hexagonal crystals, or in grains or masses; hardness 7; specific gravity 2.65; colorless when pure; luster vitreous; transparent on thin edges. Quartz is not hard to identify. Its hardness and the crystal-form separate it from most all other minerals. It is the most common mineral, making 12% of the earth’s crust. The usual crystal form is a hexagonal prism with the sides horizontally striated, and a six-sided pyramid on one or both ends. This six-sided pyramid is really two rhombohedrons, a right-handed one and a left-handed one, so that the alternate faces of the pyramid may show peculiarities, for instance three may be large and three small, as in Fig. B, Plate 30, or the alternate ones may be duller or etched in some manner. The crystals are clear and when pure colorless, but there is a tendency for some slight impurity to color them almost any hue.