Granite GEOLOGICAL CLASSIFICATION The rock forming the earth's crust falls into three generic groups: igneous, sedimentary and metamorphic. Heat, pressure and chemical reactions may change either igneous or sedimentary rock into metamorphic rock, meaning "changed in form" usually into a more compact and crystalline condition and even metamorphic rocks may be further altered to higher ranks of metamorphism. Granites usually have been classed as igneous rocks derived from molten masses or magmas, but there is wide evidence that the origin of some granites may be attributed to regional metamorphism or preexisting rocks, rearrangement and recrystallization taking place without a liquid or molten stage. The chief minerals of which granites are formed are feldspars and quartz and smaller amounts of mica and hornblende. They are classified as fine-grained, medium-grained or coarse-grained. Medium-grained granites are those in which the feldspar crystals average about one-fourth inch in diameter. If relatively coarse grained crystals appear in a fine-grained ground mass the rock is designated as porphyritic granite. A rock may have the mineral constituents of a granite but may show a banded or platy structure owing to recrystallization, folding or other changes while the rock was in a plastic or semimolten condition. Such metamorphic rocks are called granite gneisses. COLOR AND VEINING The color of a granite is governed largely by that of the feldspar, usually the most abundant mineral. However, it may be modified to some extent by the quartz, hornblende, or mica, if considerable amounts are present. Almost white, dark gray, green, light gray, pink and red granites are common. Uniform color distribution is usually a desirable feature. Dark granular igneous rocks, classified petrographically as anorthosite, basalt, diabase, diorite, or gabbro are also used as dimension stone and are classed commercially as black "granite". TEXTURE The term "texture", as applied to granite, means size, degree of uniformity and arrangement of constituent minerals. The texture of granite signifies the size and arrangement of mineral grains. Uniform grain size usually is demanded in commercial granites. Grain size varies greatly in different granites. Uniform distribution of the minerals is as important as uniform grain size. Light and dark minerals should be distributed evenly throughout the rock mass, for this gives uniform color and texture. Many commercial deposits display remarkable homogeneity; the rock may not vary in color or texture for many feet, either vertically or horizontally. THICKNESS Standard thicknesses for granite are generally 3/4 inch (2cm), 1 1/4 inches (3cm), 1 1/2 inches (4cm), 2 1/2 inches (6.4cm), 3 inches (7.6cm), and 3 1/2 inches (9cm). Metric thicknesses given above are approximate; cutting can be made to exact metric measurements through conversion of English values to metric equivalents. Note that as granite is cut thinner its tensile strength is diminished. VENEER CUTTING Quarry blocks are reduced to slabs by a gang saw or wire saw. The gang saw consists of a series of steel blades set parallel in a frame that moves forward and backward. They are fed a cutting abrasive in a stream of water. AGGLOMERATE GRANITE TILES These products are recent introductions to the industry. Typical composition is 90-95% natural granite contained in a cementitious or resinous binder. Technical details can be obtained from the manufacturer. All references in this manual relate to natural (100% granite) not man-made stone.