igneous
Dark mafic rock, possibly basalt with weathered iron staining
Likely basalt; exact identification is uncertain from the photograph
AI-generated identification · may be incorrectFine-grained, dark gray to black, with dull to slightly glassy luster and an irregular fracture; likely hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1. The rusty brown-yellow area is probably iron-oxide weathering or a small alteration vein.
Identified More igneous →
Explore Dark mafic rock, possibly basalt with weathered iron staining in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Fine-grained, dark gray to black, with dull to slightly glassy luster and an irregular fracture; likely hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1. The rusty brown-yellow area is probably iron-oxide weathering or a small alteration vein.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its age cannot be determined visually and may range from recent to billions of years.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, building stone, and occasionally as a lapidary material. This small ordinary fragment has little commercial value.
Geological facts
Basalt commonly contains plagioclase and pyroxene, with possible olivine or magnetite; it may attract a magnet weakly if magnetite-rich. A true identification would benefit from a fresh surface, streak, magnet test, and inspection for visible crystals or vesicles.
Field identification & locations
Common worldwide, especially in Hawaii, Iceland, India’s Deccan Traps, oceanic islands, and volcanic regions. The specimen’s dark color and fine texture support basalt, but slag, hornfels, or another mafic rock remain possible without testing.
