metamorphic
Serpentinized ultramafic rock with magnesite
Serpentinite, chiefly serpentine-group minerals (Mg₃Si₂O₅(OH)₄), with likely magnesite (MgCO₃) and minor chromite/magnetite
AI-generated identification · may be incorrectDark green-black, mottled, and waxy to greasy luster; the cream-white rounded area is likely magnesite or altered carbonate. Typically Mohs 2.5–4, fine-grained with no obvious cleavage; density commonly 2.5–2.7 g/cm³.
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Physical properties
Dark green-black, mottled, and waxy to greasy luster; the cream-white rounded area is likely magnesite or altered carbonate. Typically Mohs 2.5–4, fine-grained with no obvious cleavage; density commonly 2.5–2.7 g/cm³.
Formation & geological history
Forms when mantle-derived peridotite is hydrated and metamorphosed, commonly along oceanic crust and fault zones. Serpentinization is usually Paleozoic to Recent depending on the tectonic setting; age cannot be determined from the photograph.
Uses & applications
Used as decorative stone, carvings, and lapidary material; some serpentinite has been used in architecture, though asbestos-bearing varieties require caution. Common specimens generally have modest market value.
Geological facts
The dark matrix may contain magnetite, making some pieces weakly magnetic; fibrous chrysotile can occur in serpentinite. The pale material may fizz weakly in dilute hydrochloric acid if carbonate-rich.
Field identification & locations
Common in ophiolites and ultramafic terrains, including California, Italy, New Zealand, Canada, and the Appalachians. Confirm with hardness, streak, magnetism, acid reaction, and thin-section or X-ray analysis; avoid cutting unknown fibrous material.
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