Basalt

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull; Crystal structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0.

Formed from the rapid cooling of magnesium-rich and iron-rich lava (basaltic lava) at or very near the Earth's surface. It can be found in volcanic oceanic crust, mid-ocean ridges, and continental flood basalts of various geological ages from the Archean to the present.

Used extensively in construction as crushed stone for road base, railroad ballast, and concrete aggregate. It is also used as dimension stone for tiles and monuments, and spun into rock wool for thermal insulation.

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floors. It is also found on other celestial bodies like the Moon (lunar maria), Mars, and Venus.

Identify in the field by its high density, dark color, and lack of visible individual crystals without a lens. Look for a fine-grained, 'sugary' texture and occasional small, green olivine crystals. Common in volcanic regions such as Iceland, Hawaii, and the Pacific Northwest of the USA.