Basalt

Hardness: 6 (feldspar component), Color: Dark gray to black, Luster: Dull to vitreous, Crystal structure: Aphanitic (fine-grained), Cleavage: Absent, Specific Gravity: 2.7-3.0

Basalt forms from the rapid cooling of basaltic lava exposed at or very near the Earth's surface. It's often associated with volcanic activity (both continental and oceanic), mid-ocean ridges, and hot spots. It is one of the most common rocks on Earth, forming much of the oceanic crust. Its age varies widely from ongoing eruptions to billions of years old.

Construction material (road aggregate, concrete), dimension stone, insulation (as rock wool), used in sculpture and art. Industrially, it is crushed for road base and paving material. Due to its durability, it's also used for railway ballast. Not typically used in jewelry due to its opaque nature and common occurrence, but polished basalt can be used for decorative items.

Basalt is the most abundant bedrock on Earth, forming the majority of the oceanic crust. The surface of the Moon's maria, and parts of Mars and Venus are also composed of basalt. Iconic geological features like the Giant's Causeway in Northern Ireland and the columnar jointing at Devil's Postpile in California are made of basalt. Large igneous provinces, such as the Deccan Traps in India, are primarily composed of basalt.

To identify basalt in the field, look for its dark color (typically black or dark gray) and fine-grained texture (individual mineral crystals are usually not visible to the naked eye, indicating rapid cooling). It can sometimes exhibit vesicular textures (small holes left by gas bubbles) or columnar jointing. Common locations include volcanic regions, ocean floors, and large flood basalt provinces worldwide. When broken, it typically has a conchoidal or hackly fracture. It is dense and will feel heavy for its size. Fresh surfaces may appear slightly glassy, but weathered surfaces are often dull. Can be confused with obsidian, but obsidian is volcanic glass and will break with sharper, more purely conchoidal fractures and is typically shinier.