Granite

Hardness: 6-7 on Mohs scale (due to quartz and feldspar). Color: Highly variable, but typically speckled with white, pink, gray, and black minerals. Luster: Vitreous to sub-vitreous. Crystal Structure: Phaneritic (coarse-grained, visible crystals), typically anhedral to subhedral (irregular to somewhat formed crystals). Cleavage: Feldspar components have good cleavage, quartz does not. Specific Gravity: 2.6 – 2.7 g/cm³.

Granite forms from the slow crystallization of magma below Earth's surface (intrusive). The cooling process can take millions of years, allowing large mineral crystals to grow. It is typically associated with mountain-building events (orogenies) and is common in continental crust. Geological age varies widely, from Precambrian (billions of years old) to Cenozoic (tens of millions of years old).

Industry: Widely used as building stone, dimension stone, and for paving. Construction: Countertops, flooring, wall tiles, monuments, curbstones. Jewelry: Not typically used as a gemstone, but polished slabs are used in decorative items. Other: Often crushed and used as aggregate in road construction and concrete.

Granite is one of the most abundant intrusive igneous rocks in the Earth's continental crust. Mount Rushmore, the famous American monument, is carved into a granite batholith. 'Granite' is often used commercially to refer to any coarse-grained ingeous rock composed of interlocking crystals, which may or may not be true geological granite.

Identify in the field: Look for its speckled appearance with interlocking, visible mineral crystals. The most common minerals will be clear/smoky quartz, pink/white feldspars, and dark biotite or amphibole. It's a hard rock that won't scratch easily with a knife. Common Locations: Found in continental shield areas, ancient mountain ranges, and large batholiths worldwide. Major producers include Brazil, China, India, Italy, and the United States (e.g., Vermont, North Carolina). Tips for collectors: Granite often weathers into rounded boulders or outcrops. Look for areas where erosion has exposed the rock. Riverbeds and glacial till can also contain well-rounded granite pebbles like the one in the image.