metamorphic
Serpentinite (serpentinized ultramafic rock)
Serpentinite; chiefly serpentine-group minerals (Mg₃Si₂O₅(OH)₄), commonly with magnetite, brucite, and relict olivine/pyroxene
AI-generated identification · may be incorrectTypically mottled olive-green, gray, black, and cream with waxy to greasy luster; hardness about 2.5–4 Mohs, massive texture, poor cleavage, and relatively high density (~2.5–2.7). The specimen’s pale veining and dark green patches are consistent with serpentinization, though a photo cannot exclude talc-rich ultramafic…
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Physical properties
Typically mottled olive-green, gray, black, and cream with waxy to greasy luster; hardness about 2.5–4 Mohs, massive texture, poor cleavage, and relatively high density (~2.5–2.7). The specimen’s pale veining and dark green patches are consistent with serpentinization, though a photo cannot exclude talc-rich ultramafic rock or soapstone.
Formation & geological history
Forms when mantle-derived peridotite is hydrated and altered by hydrothermal fluids, commonly along oceanic crust and plate-boundary fault zones. Most examples are Paleozoic to Mesozoic or younger, depending on the tectonic setting.
Uses & applications
Used as decorative and architectural stone, ornamental carvings, and historically as a source rock associated with asbestos minerals; cutting or sanding should be avoided until tested for fibrous asbestos.
Geological facts
Often contains magnetite, making some pieces weakly magnetic; polished surfaces can show striking green veining. Serpentinite is California’s state rock.
Field identification & locations
Identify by its green-black mottling, softness, waxy feel, curved fractures, and possible magnetism; common in ophiolite belts worldwide, including California, Italy, New Zealand, and the Appalachians.
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