Most minerals grow into distinctive shapes if they are free to grow. A familiar example is the formation of salt crystals that grow on a saucer of evaporating salt water. The distinctive shapes of crystals are called their habits, and the flat surfaces that develop are called crystal faces, the angles of which may be used to identify the mineral. The individual atoms of a crystal always arrange themselves in the same way, so that each mineral breaks characteristically. Some minerals break more easily in particular directions and present a flat, smooth surface. This characteristic is called cleavage and the cleavage surfaces, although sometimes confused with crystal faces, may be useful for identifying a particular mineral. The manner in which a mineral breaks when the broken surface does not include cleavage surfaces is called its fracture, and this too may give a clue to the identification of the mineral. There are other physical features useful in identifying minerals. Some of them, such as color, are quite obvious. The color of the powder left when a mineral is scraped on a rough, white surface, such as unglazed porcelain or tile, is called streak. Luster refers to the brightness of light reflected from the mineral’s surface. Transparency and translucency refer to the mineral’s ability to transmit light, and tenacity is a measure of its toughness. Two special physical characteristics of minerals are important to their identification—specific gravity and hardness. Specific gravity simply means the ratio of the weight of the mineral to an equivalent volume of water. For example, if a mineral has a specific gravity of 4, then a cubic inch of the mineral weighs as much as 4 cubic inches of water. Hardness is measured by the ability of one mineral to scratch another, and a set of ten standard minerals has been selected for determining this characteristic. The listing below, from soft to hard, is known as Mohs scale. 1—Talc 2—Gypsum 3—Calcite 4—Fluorite 5—Apatite 6—Orthoclase 7—Quartz 8—Topaz 9—Corundum 10—Diamond A rough measure of hardness can be made by using handy objects. Your fingernail has a hardness ranging from 2 to 3, a penny is a little harder than 3, window glass ranges from less than 5 to approximately 6 in hardness, and a knife blade is generally in the range of 5 to 6. ROCKS Rocks, being mixtures of minerals, are more complex than minerals and are therefore classified in a more complicated way. The broadest grouping of rocks is based on the origin of the rock rather than on the minerals that compose it. In this scheme all rocks are divided into three general groups, igneous, sedimentary, and metamorphic.