igneous
Dark porphyritic volcanic rock, likely andesite or basalt
Fine-grained plagioclase–pyroxene rock; exact identification requires testing
AI-generated identification · may be incorrectHardness about 6–7 Mohs; dark gray to purplish-brown, dull to slightly waxy luster, with faint lighter mineral patches. Massive, nonfoliated texture and no visible cleavage; likely specific gravity around 2.7–3.0.
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Physical properties
Hardness about 6–7 Mohs; dark gray to purplish-brown, dull to slightly waxy luster, with faint lighter mineral patches. Massive, nonfoliated texture and no visible cleavage; likely specific gravity around 2.7–3.0.
Formation & geological history
Formed when lava cooled relatively quickly at or near Earth’s surface, probably during the Cenozoic or older volcanic activity; age cannot be determined visually. The rounded shape indicates stream or beach abrasion, or tumbling.
Uses & applications
Used as aggregate, road stone, riprap, and occasionally landscaping material; generally little gemstone value. Attractive polished pieces may be sold as common lapidary specimens.
Geological facts
The pale patches may be plagioclase feldspar, while dark areas may contain pyroxene, amphibole, or magnetite. A streak, magnet test, and fresh-break examination would help distinguish andesite from basalt.
Field identification & locations
Common in volcanic regions worldwide, including the Cascades, Andes, Japan, Iceland, and Hawaii. Look for fine grain, very low porosity, and possible magnetic response; a dilute acid test should show no fizz.
