Granite

Hardness: 6-7 (Mohs scale) for quartz and feldspar; Color: Variable, typically light gray to dark gray with speckles of light (quartz, feldspar) and dark (biotite, hornblende) minerals. The image shows a dark gray rock with white/shiny speckles; Luster: Vitreous to dull depending on the minerals present; Crystal Structure: Phaneritic (coarse-grained) meaning individual crystals are visible to the naked eye. In the image, small, visible crystals are apparent; Cleavage: Visible cleavage planes in feldspar and mica, but not dominant for the rock as a whole.

Granite forms from the slow crystallization of magma below the Earth's surface. This slow cooling allows for the formation of large, visible mineral grains. It is primarily formed in continental crust settings where tectonic plates collide, leading to the melting of rocks and the intrusion of magma. Its geological age varies widely depending on the specific location and tectonic events, ranging from Precambrian to relatively recent Cenozoic formations.

Industry: Widely used as building material for countertops, floor tiles, paving stones, and dimension stone due to its durability, abundance, and attractive appearance. Construction: Used in various outdoor applications like curbing, building facades, and monuments. Jewelry: Not typically used as a gemstone itself, but polished granite slabs are used decoratively.

Granite is one of the most abundant intrusive igneous rocks in the Earth's continental crust. Mount Rushmore is carved from granite. It's often associated with mountain-building events and the formation of continents. The term 'granite' is often used loosely in commercial contexts for any coarse-grained igneous rock.

Field Identification: Look for a coarse-grained rock with interlocking crystals of varying colors, typically shades of gray, white, pink, and black. Identify prominent minerals like white/pink feldspar, clear/smoky quartz, and black mica (biotite) or amphibole. The sample in the image, with its visible light-colored crystals set in a darker matrix, is consistent with granite. Common Locations: Found globally in mountain ranges, continental shield areas, and ancient cratons. Major occurrences are in the Sierra Nevada Batholith (USA), Scottish Highlands, and parts of Scandinavia, South America, and Africa. Tips for Collectors: Granite is very common and easily identifiable. Look for samples that display a good mix of its constituent minerals and interesting textures.