igneous
Fine-grained gray rock, likely andesite
Andesite (intermediate volcanic rock; exact identification requires testing)
AI-generated identification · may be incorrectGray-green, dull, fine-grained and massive, with a few pale mineral grains; typical andesite is composed mainly of plagioclase with pyroxene or amphibole. Mohs hardness is roughly 6–7; crystals are too fine to see clearly, and there is no obvious cleavage.
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Physical properties
Gray-green, dull, fine-grained and massive, with a few pale mineral grains; typical andesite is composed mainly of plagioclase with pyroxene or amphibole. Mohs hardness is roughly 6–7; crystals are too fine to see clearly, and there is no obvious cleavage.
Formation & geological history
Likely cooled from intermediate-composition lava at or near the surface, commonly in volcanic arcs. Andesite forms over geologic time wherever volcanism occurs; the specimen’s age and source cannot be determined from the photo.
Uses & applications
Andesite is used as crushed aggregate, roadstone, and building stone; attractive pieces may be kept as hand specimens.
Geological facts
Its muted color and fine texture are consistent with volcanic rock, but other gray rocks can look similar. A hand specimen alone cannot establish the identification conclusively.
Field identification & locations
Look for a dense, fine-grained texture and scattered visible feldspar crystals; compare with nearby rocks and use a fresh surface, hardness, and mineral tests for confirmation. Andesite is common in volcanic mountain belts, including the Andes, Japan, and the Cascades.
