All substances found in the earth or upon its surface, which are not the products of art or of organic life, are regarded by the mineralogist as _simple minerals_. About four hundred mineral species are known, and the varieties are much more numerous; but only a small number of them are so abundant as to claim the attention of the geologist. An acquaintance with the following species is, however, necessary. _Quartz_ is probably the most abundant mineral in nature. It is composed wholly of silex. Its specific gravity is 2.65. It is the hardest of the common minerals, gives sparks with steel, scratches glass, and breaks into irregular angular fragments under the hammer. When crystallized, its most common form is that of a six-sided prism, terminated by six-sided pyramids. When pure, it is transparent or translucent, and its lustre is highly vitreous. The transparent variety is called _rock crystal_. When purple, it is _amethyst_. When faint red, it is _rose quartz_. When its color is dark brown, or gray, and it has a conchoidal fracture, it is _flint_. When quartz occurs in white, tuberous masses, of a resinous lustre and conchoidal fracture, it is _opal_. The precious opal is distinguished by its lively play of colors. _Jasper_ is opaque, and contains a small per cent, of oxide of iron, by which it is colored dull red, yellowish red or brown. The light-colored, massive, translucent variety is _chalcedony_. The flesh-colored specimens are _carnelian_. When composed of layers of chalcedony of different colors, it becomes _agate_. Several of the varieties of quartz, such as amethyst, opal, carnelian and agate, are used to considerable extent in jewelry. _Felspar_ is composed of silex, alumina and potassa. It resembles quartz, but it is not as hard, cleaves more readily, and is not generally transparent. Its specific gravity is 2.47. Its lustre is feebly vitreous, but pearly on its cleavage faces. Its color is sometimes green, but generally dull white, and often inclined to red or flesh-color. _Mica_ is composed of the same ingredients as felspar, together with oxide of iron. Its specific gravity is nearly three. It is often colorless, but frequently green, smoky, or black. It may be known by its capability of division into exceedingly thin, transparent, elastic plates. _Hornblende_ is composed of silex, alumina and magnesia. Its specific gravity is a little above three. Its color is generally some shade of green. When dark green or black, whether in a massive or crystalline state, it is _common hornblende_. When light green, it is _actinolite_. The white variety is _tremolite_. When it is composed of flexible fibres, it is _asbestus_; and when the fibres have also a silky lustre, it is _amianthus_.