Stone dealers lump a wide variety of rock types under the name "granite." Commercial granite is any (1) crystalline rock that is (2) harder than marble (3) with large mineral grains. Let's unpack that statement:
- Crystalline rock is rock that consists of mineral grains that are tightly intergrown and locked together, making a tough, impervious surface. Crystalline rocks are made of grains that have grown together at high temperature and pressure, rather than being made of existing sediment grains that have been cemented together under gentler conditions. That is, they are igneous or metamorphic rocks rather than sedimentary rocks. This differentiates commercial granite from commercial sandstone and limestone.
- Marble is a crystalline, metamorphic rock, but one that consists largely of the mineral calcite. Granite does not contain calcite but instead consists of much harder minerals, mostly feldspar and quartz. This differentiates granite from commercial marble and travertine.
- Commercial granite has all its minerals visible as individual grains, which is the origin of the word "granite." This differentiates granite from commercial slate, greenstone and basalt.