Mineralogy, the science which treats of minerals, does not embrace all that relates to the mineral kingdom. Simple minerals alone, or homogeneous mineral substances, are regarded as the subjects of mineralogy; rocks formed by the aggregation of simple minerals, and their relations to each other, are the subjects of Geology (q.v.). This limitation of the term mineralogy is comparatively recent. Geology or geognosy was formerly included in it. The arrangement and description of simple minerals according to their external characters has been called by Werner and others Oryetognosy, but the term has fortunately fallen into disuse. Nor is the study of mere external characters sufficient in mineralogy. The chemical composition of minerals equally demands attention. In the classification of minerals some mineralogists, as Mohs and Jameson, have regarded only the external characters, and some, as Berzelius, only the chemical composition; but the results have been unsatisfactory, and the present tendency is in favour of a system which seeks to constitute natural groups by having regard to both.
Some minerals being of great use, and others highly valued for their beauty, have received much attention from the earliest ages. But the ancient naturalists describe few minerals. The first attempt at scientific mineralogy was by George Agricola in the 16th century. The systems of the Swedes Wallerius and Cronstedt, in the later half of the 18th century, were the first worthy of the name. That of Werner followed, and was extensively adopted. The discoveries of Haüy in crystallography, and the progress of chemistry, gave mineralogy a new character; and then sprang up two schools of mineralogists, one resting chiefly on external characters, and the other on chemical composition.
The chemical classification of minerals is rendered difficult by the endless variety of combination and proportion in the elements of which they are composed, the presence of substances not essential to the mineral, and yet more or less affecting its characters, and the frequent impossibility of determining what is to be deemed essential and what accidental. Chemical purity is almost never found in nature. Even the purest diamond, when burned, leaves some traces of ash; and the various colours of diamond, quartz, and other minerals are due to the presence of substances which are often in so small quantity as not to affect their crystalline forms or other physical properties. Again, some minerals of identical chemical composition differ in their crystallisation, so that an arrangement founded upon it would separate them too widely. There are also many minerals which are often found in an uncrystallised state, and others which are always so. In the arrangement of minerals into natural groups, their chemical composition, although not alone to be regarded, is of the first importance, so that the place of a new mineral in the system can never be determined without analysis; and in determining the nature of a mineral chemical tests, such as the application of acids, are continually resorted to. It is also necessary to know its specific gravity, and how it is acted upon both by a moderate heat and by the blowpipe. An examination of the crystalline forms, with measurement of the angles of the crystals, is often sufficient to distinguish minerals which have otherwise much resemblance. The cleavage of crystals is also important—a readiness to split in planes parallel to certain of their faces only, by which the primitive form of the crystal may be ascertained. Minerals not crystallised exhibit important varieties of structure, as laminated, fibrous, granular, &c. Certain peculiarities of form are also frequently characteristic of uncrystallised minerals, as mamillary, botryoidal, &c. Minerals exhibit, when broken, very different kinds of fracture, as even, conchoidal, splintery, &c. Opaqueness, translucency, and transparency are more or less characteristic of different kinds: electric and magnetic properties demand attention; and very important characters are derived from lustre, which in some minerals is metallic, in others semi-metallic, in others pearly, vitreous, &c. Colour is not generally of much importance, but in some minerals it is very characteristic. The colour of the powder formed when a mineral is scratched often differs from that of the solid mass. This is the streak of the mineral, and is frequently very characteristic. Hardness and tenacity are very important, and are of all various degrees. Unctuousity and other peculiarities to be ascertained by the touch are very characteristic of some minerals, and peculiarities of taste and smell belong to others.
Mineralogy has very important relations with geology, which cannot be studied without regard to the mineral constituents of rocks. The mineral composition of soils greatly affects vegetation and agriculture. The economical uses of minerals are also very important and various. It is enough merely to allude to salt, sulphur, borax, alum, graphite, cryolite, native metals, metallic ores, &c. Naphtha, petroleum, bitumen, asphalt, &c. are of well-known utility; and a high value has always been attached to gems and other ornamental stones. There are special handbooks by Bauerman, Dana, Wöhler, Brush, and Erni. See GEOLOGY, CRYSTALLOGRAPHY.