metamorphic
Tumbled serpentine with dark magnetite/chromite matrix
Serpentinite; chiefly antigorite/lizardite, Mg3Si2O5(OH)4, with magnetite and possibly chromite
AI-generated identification · may be incorrectTypically pale green to yellow-green, waxy to greasy luster, Mohs 2.5–4, opaque to translucent, with irregular dark magnetic inclusions; massive rather than visibly crystalline and lacking strong cleavage.
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Physical properties
Typically pale green to yellow-green, waxy to greasy luster, Mohs 2.5–4, opaque to translucent, with irregular dark magnetic inclusions; massive rather than visibly crystalline and lacking strong cleavage.
Formation & geological history
Formed when ultramafic mantle rocks, especially peridotite, were hydrated during low-temperature metamorphism. Serpentinite commonly formed in oceanic crust and subduction zones, generally from Paleozoic to modern tectonic settings; this piece is polished/tumbled.
Uses & applications
Used as ornamental stone, carvings, cabochons, and inexpensive decor; historically used as a marble substitute. Avoid prolonged heating or dust inhalation because some serpentinite sources may contain asbestos-forming minerals.
Geological facts
The dark material is likely magnetite or chromite-rich relic ultramafic rock, explaining the strong black veining and possible magnetism. Serpentine is California’s state rock.
Field identification & locations
Identify by its low hardness, waxy feel, pale green/cream color, and association with black magnetic streaks; a knife may scratch it. Common in California, the Appalachians, New Zealand, Italy, and Japan.
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