Granite

Hardness: 6-7 (Mohs scale), Color: Typically speckled with white, pink, gray, and black minerals. The image shows a sample with prominent reddish-pink (feldspar) and dark green/black (biotite/hornblende) grains. Luster: Vitreous to glassy (quartz, feldspar), dull to sub-metallic (mafic minerals). Crystal structure: Phaneritic (visible crystals), often interlocking. Cleavage: Feldspar exhibits good cleavage in two directions; quartz has no cleavage. Specific Gravity: 2.6 – 2.7 g/cm³.

Formation process: Granite is an intrusive igneous rock, meaning it forms from the slow crystallization of magma below Earth's surface. This slow cooling allows for the formation of large, visible mineral grains. Geological age: Granites can be found from the Precambrian era (over 540 million years ago) to relatively recent geological periods, depending on the specific tectonic activity. Environments: Formed in continental crust settings, often associated with orogenic (mountain-building) belts and subduction zones.

Industry/Construction: Widely used as a dimension stone for building facades, floor tiles, countertops, paving, and monuments due to its hardness, durability, and attractive appearance. Jewelry: Not typically used in jewelry due to its opaque and granular nature, but polished granite can be used in decorative items. Collecting Value: Common and widespread, so individual specimens generally don't have high monetary value, but large, well-preserved samples are prized by collectors for their aesthetic and geological interest.

Granite is one of the most widely recognized igneous rocks and forms the core of many mountain ranges. Large granite intrusions are known as batholiths. It's often associated with continental crust formation and collision zones.

How to identify in the field: Look for its characteristic coarse-grained texture and the presence of quartz (glassy, colorless to gray), feldspar (pink, white, or gray, often blocky), and dark minerals like biotite (black, flaky) or hornblende (dark green to black, prismatic). It resists weathering well and often forms prominent outcrops. Common locations: Found globally in continental interiors and mountain ranges, notably the Sierra Nevada batholith in the USA, Scottish Highlands, parts of Scandinavia, and the Canadian Shield. Tips for collectors: Granite is very durable, so samples are often robust. Look for samples with clearly identifiable mineral grains and interesting color variations. Smaller, rounded pieces like the one pictured are often found in riverbeds or glacial tills, indicating erosion and transport.