metamorphic
Tumbled greenstone, likely serpentinite
Serpentinite (predominantly serpentine-group minerals: Mg₃Si₂O₅(OH)₄)
AI-generated identification · may be incorrectDark green, opaque, finely mottled, and waxy to dull with a smooth tumbled surface; hardness about 2.5–4 Mohs, non-cleavable to weakly cleaved, and relatively dense (specific gravity ~2.5–2.7). The image is consistent with serpentinite, though visual identification alone cannot exclude nephrite or another greenstone.
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Physical properties
Dark green, opaque, finely mottled, and waxy to dull with a smooth tumbled surface; hardness about 2.5–4 Mohs, non-cleavable to weakly cleaved, and relatively dense (specific gravity ~2.5–2.7). The image is consistent with serpentinite, though visual identification alone cannot exclude nephrite or another greenstone.
Formation & geological history
Forms when ultramafic mantle rocks such as peridotite are hydrated and metamorphosed, commonly along oceanic crust and subduction-related faults; specimens may range from Paleozoic to modern tectonic settings.
Uses & applications
Used as a decorative stone, carvings, beads, cabochons, and tumbled specimens; some varieties of greenstone are sold as inexpensive jewelry material.
Geological facts
Serpentinite often contains magnetite and may show black speckling or veining. It is California’s official state rock and commonly forms slick, polished-looking exposures in fault zones.
Field identification & locations
Look for a soft, waxy feel, mottled green color, black magnetite grains, and a scratchability greater than quartz; a steel knife may scratch it, while glass should not. Common localities include California, Italy, New Zealand, Canada, and the Ural Mountains.
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