igneous

Basalt

Basalt (Aphanitic Mafic Volcanic Rock)

Hardness: 6 on Mohs scale; Color: typically dark grey to black (weathering to brown or rust); Luster: dull to sub-metallic; Structure: fine-grained (aphanitic), often with small visible crystals of plagioclase or olivine; Cleavage: none (fractures conchoidally or irregularly); Specific Gravity: 2.8 - 3.0.

Hardness: 6 on Mohs scale
Color: typically dark grey to black (weathering to brown or rust)
Luster: dull to sub-metallic

Identified March 12, 2026More igneous →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or moon. Most basalt on Earth formed within the last 200 million years, though specimens can date back billions of years. Common in oceanic crust and shield volcanoes.

Uses & applications

Extensively used in construction as an aggregate for road base, concrete, and asphalt. High-quality basalt (scoria) is used in landscaping and as thermal insulation (rock wool). Occasionally used for stone tools or small carvings.

Geological facts

Basalt is the most common rock type in Earth's crust and makes up most of the ocean floor. It is also found on the Moon and Mars—the dark plains on the moon (maria) are actually giant basaltic lava flows.

Field identification & locations

Identify in the field by its high density, dark color, and fine-grained texture lacking visible quartz. Often feels heavy for its size. Common in volcanic regions such as Iceland, Hawaii, and the Pacific Northwest. Collectors look for pieces containing green olivine (peridot) crystals.

Other igneous specimens

See all

Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous