igneous
Tumbled dark stone, likely basalt or andesite
Fine-grained mafic/intermediate volcanic rock; commonly basalt (plagioclase ± pyroxene, olivine, and iron oxides)
AI-generated identification · may be incorrectDark olive-brown to charcoal, smooth rounded surfaces, and dull to faintly vitreous luster. Typical hardness is about 5–6 Mohs; massive, with no visible cleavage or crystals; density commonly 2.7–3.1 g/cm³.
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Physical properties
Dark olive-brown to charcoal, smooth rounded surfaces, and dull to faintly vitreous luster. Typical hardness is about 5–6 Mohs; massive, with no visible cleavage or crystals; density commonly 2.7–3.1 g/cm³.
Formation & geological history
Formed when volcanic lava cooled rapidly at or near Earth’s surface, then was naturally or mechanically tumbled and polished. The rock may range from ancient to geologically recent, depending on its source.
Uses & applications
Used as landscaping aggregate, roadstone, building stone, and decorative tumbled material; generally not a jewelry gemstone except as an inexpensive cabochon or craft stone.
Geological facts
The olive coloration may reflect weathering, chlorite, or iron-bearing minerals. A precise identification requires a streak, hardness, magnetism, and ideally thin-section or chemical testing.
Field identification & locations
Common in volcanic regions and stream gravels worldwide, especially Hawaii, Iceland, the western Americas, and oceanic terrains. Look for very fine grain, high density, dark streak, and lack of acid reaction; distinguish from glass by its opaque, non-conchoidal interior.
