Granite

Hardness: 6-7 (Mohs scale) due to quartz and feldspar content. Color: Highly variable, often speckled or mottled with shades of grey, black, white, and sometimes pink or green, depending on mineral composition. Luster: Vitreous to dull, depending on the individual mineral grains. Crystal Structure: Crystalline, typically phaneritic (coarse-grained) meaning individual crystals are visible to the naked eye. Cleavage: Exhibits cleavage planes based on the mineral components, e.g., feldspar has two cleavage directions. Specific Gravity: Typically 2.6-2.7 g/cm³.

Granite is an intrusive igneous rock, meaning it formed from the slow crystallization of magma below Earth's surface. It forms from felsic magma (rich in silica and alkali metals) that cools slowly at great depths, allowing large crystals to grow. This slow cooling can take millions of years, leading to its characteristic coarse-grained texture. It is associated with continental collision zones and is found in the cores of mountain ranges and stable continental cratons. Its age varies widely from Precambrian to Cenozoic.

Industry: Widely used as a construction material for building facades, paving, and monuments due to its strength, durability, and resistance to weathering. Jewelry: Polished granite slabs are used for countertops, floor tiles, and decorative panels. While not typically a jewelry stone itself, some unique granite varieties with specific mineral inclusions can be cut for ornamental purposes. Construction: A primary material for curbstones, building blocks, and aggregate in road construction.

Granitoids, the broader group to which granite belongs, make up a significant portion of continental crust. Famous natural formations include Yosemite's Half Dome and Mount Rushmore, both carved out of granite. The term 'granite' is often used commercially for any phaneritic igneous rock composed primarily of quartz and feldspar, even if it's technically granodiorite or monzonite by geological classification. It is one of the most durable and long-lasting building materials.

Identify in the field: Look for its coarse-grained (phaneritic) texture, where individual interlocking mineral grains (quartz, feldspar, biotite/hornblende) are clearly visible. It is typically hard, resisting scratches from a knife. The color is usually light, often mottled with dark specks. It does not exhibit banding typically seen in metamorphic rocks like gneiss. Common locations: Granite makes up large parts of continental crust and is found globally. Major granite exposures can be found in mountain ranges (e.g., Sierra Nevada in the USA, Scottish Highlands), tectonically stable areas (e.g., Canadian Shield, Scandinavian Shield), and ancient cratons. Tips for collectors: Granite typically forms large masses, so finding small, attractive, weathered samples with well-defined crystals is common in riverbeds, glacial tills, and outcrops. Its hardness means it resists abrasion, making river-worn pebbles very smooth.