[Illustration: Fig. 75.--Crystal forms cf common minerals: a, galena (Isometric); b, sphalerite (Isometric); c, beryl (Hexagonal); d, hematite (Hexagonal); e, magnetite (Isometric); f, barite (Orthorhombic); g, apatite (Hexagonal); h. sulphur (Monoclinic); i, gypsum (Monoclinic); j, chalcopyrite (Tetragonal); k, fluorite (Isometric); l, zircon (Tetragonal); m, tourmaline (Trigonal); n, corundum (Hexagonal).] Beryl. A silicate of aluminum and the rare chemical element beryllium. Hexagonal crystals usually of very simple six-sided prismatic habit (see Figure 75c). Color white, green, blue, or yellow. Specific gravity, 2.8. Cleavage practically absent. It is a very exceptionally hard mineral, being 8 in the scale. Very large crystals have been found, as, for example, in New Hampshire, where single crystals several feet long weigh a ton or more. Beryl is also of special interest because two of its varieties--_emerald_ (green) and _aquamarine_ (blue)--are well-known gem stones, the emerald being one of the most highly prized gems. The colors are due to slight impurities. Beryl most commonly occurs in dikes of coarse granite called pegmatite, but also in certain metamorphic and sedimentary rocks. Calcite. Commonly called "calc spar." A carbonate of lime. Hexagonal crystals in a great variety of forms, but all with crystal faces arranged in sixes around the principal or vertical axis forming rhombohedrons, prisms, or double-pointed pyramids. The principal axis of symmetry is sixfold by a combination of rotation and reflection. Very perfect cleavages in three directions yielding fragments whose faces make angles of 75 and 105 degrees. Color, white when pure, but variously colored when impure. Hardness, 3 (very easily scratched by a knife); specific gravity, 2.7. Calcite is a very common mineral, especially in limestone (including _chalk_) and marble which are usually largely made up of it. Also commonly found in veins, and as spring and cave deposits (stalactites). A porous, stringy variety, called _travertine_, is deposited by certain hot springs, as at Mammoth Hot Springs in Yellowstone Park. A very transparent crystalline variety is called _Iceland spar_. Calcite is a very useful mineral. Limestone and marble are widely used as a building stone, and for decorative purposes, statuary, etc. Limestone is burned for quicklime, used as a flux in smelting certain ores, in glass making, etc. Cassiterite. The one great ore of tin whose composition is oxide of tin. Tetragonal crystallization (Figure 73c). Hardness greater than steel, being over 6 in the scale. Specific gravity 7, which is notably high. Color, brown to nearly black. Cleavage, practically absent. Fairly widespread in small amounts, and in commercial quantities in only a few localities, usually in veins in granite or metamorphic rocks near granite, as at Cornwall, England, also in the form of rounded masses in gravel deposition as in the Malay region.