igneous
Tumbled basalt (likely)
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, often with minor olivine and iron oxides
AI-generated identification · may be incorrectDark charcoal to purplish-brown, dull to weakly vitreous luster, opaque, and fine-grained; typically Mohs 6–7 with no visible cleavage and specific gravity around 2.8–3.0. The smooth rounded surface indicates tumbling or natural abrasion.
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Physical properties
Dark charcoal to purplish-brown, dull to weakly vitreous luster, opaque, and fine-grained; typically Mohs 6–7 with no visible cleavage and specific gravity around 2.8–3.0. The smooth rounded surface indicates tumbling or natural abrasion.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and lava flows. Individual pieces may range from recent volcanic material to billions of years old, depending on source.
Uses & applications
Used as aggregate, roadstone, riprap, railroad ballast, and occasionally as decorative tumbled stone or carving material. Small polished pieces generally have modest collector value.
Geological facts
The purplish coloration may result from iron oxidation, weathering, or included minerals; precise identification requires a fresh surface or testing. Basalt is the most abundant volcanic rock on Earth.
Field identification & locations
Identify by its dark color, heavy feel, fine texture, and lack of obvious crystals; a streak, magnetism test, or close inspection may distinguish it from andesite, slag, or jasper. Common worldwide, especially near volcanic regions and oceanic settings.
