igneous
Likely tumbled basalt or dark volcanic rock
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or iron oxides
AI-generated identification · may be incorrectDark gray-black, fine-grained, subvitreous to dull luster, rounded by tumbling or water; hardness about 5–6 Mohs, no visible cleavage, and typically dense (specific gravity ~2.8–3.0). The image is too small to confirm mineral grains or texture.
Identified More igneous →
Explore Likely tumbled basalt or dark volcanic rock in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray-black, fine-grained, subvitreous to dull luster, rounded by tumbling or water; hardness about 5–6 Mohs, no visible cleavage, and typically dense (specific gravity ~2.8–3.0). The image is too small to confirm mineral grains or texture.
Formation & geological history
Basalt forms when low-silica basaltic lava rapidly cools at or near Earth’s surface, commonly in volcanic regions and oceanic crust; specimens may range from recent eruptions to billions of years old. Its rounded shape suggests natural abrasion or human tumbling.
Uses & applications
Used as crushed aggregate, road stone, railway ballast, dimension stone, and occasionally in landscaping or jewelry as an inexpensive polished stone. Collector value is generally low.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, magnetic response, or tiny pale feldspar crystals can help distinguish it from other dark stones.
Field identification & locations
Common worldwide, especially Hawaii, Iceland, the Deccan Traps, and oceanic volcanic terrains; test for a weak magnet, dense feel, and fine texture. Estimated value: $1–$10 for a small common tumbled piece; unusual textures or provenance may reach $20–$50.
