igneous
Tumbled dark-gray stone, likely basalt
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and sometimes olivine
AI-generated identification · may be incorrectTypically Mohs 5–6; dark gray to black, dull to subtly vitreous, with a compact fine-grained texture and no visible cleavage. The rounded, scratched surface indicates tumbling or natural abrasion; exact identification requires testing.
Identified More igneous →
Explore Tumbled dark-gray stone, likely basalt in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Typically Mohs 5–6; dark gray to black, dull to subtly vitreous, with a compact fine-grained texture and no visible cleavage. The rounded, scratched surface indicates tumbling or natural abrasion; exact identification requires testing.
Formation & geological history
Forms when mafic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Basalt ranges from Precambrian to modern age and is continuously forming today.
Uses & applications
Used as aggregate, roadstone, riprap, paving, and dimension stone; polished or tumbled pieces are sold as inexpensive decorative stones. Typical value is about $1–$10 for a small common tumbled specimen.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Look for high density, dark color, fine grain, and resistance to scratching; it should not fizz with dilute hydrochloric acid. Common worldwide, especially Hawaii, Iceland, India, the oceanic crust, and volcanic regions.
