metamorphic
Greenstone, likely serpentinite
Serpentinite; chiefly serpentine-group minerals (Mg₃Si₂O₅(OH)₄), possibly with magnetite and chlorite
AI-generated identification · may be incorrectDark green to black, mottled and fine-grained with a waxy to subvitreous luster; typically Mohs 2.5–4, massive rather than visibly crystalline, with uneven fracture and no strong cleavage. The pale speckling may indicate altered mafic minerals or veins.
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Physical properties
Dark green to black, mottled and fine-grained with a waxy to subvitreous luster; typically Mohs 2.5–4, massive rather than visibly crystalline, with uneven fracture and no strong cleavage. The pale speckling may indicate altered mafic minerals or veins.
Formation & geological history
Forms when ultramafic mantle rocks such as peridotite are hydrated during seafloor spreading or tectonic metamorphism, commonly in ophiolite belts; individual specimens may range from Paleozoic to recent tectonic settings.
Uses & applications
Used as a decorative and carving stone, dimension stone, and collector material; historically used for beads and ornamental objects. Avoid prolonged handling or dust exposure if asbestos-form serpentine is suspected.
Geological facts
Serpentinite can contain fibrous chrysotile, a naturally occurring asbestos mineral, although not every specimen does. Its green color commonly results from serpentine minerals and iron-bearing alteration products.
Field identification & locations
Likely identification requires a waxy feel, low hardness, green-black color, and absence of obvious quartz grains; a steel nail may scratch it. Common in California, the Appalachians, the Alps, New Zealand, and other ophiolite zones; acid testing is generally unhelpful.
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