metamorphic
Tumbled serpentinite (likely antigorite-rich serpentine)
Serpentinite; chiefly serpentine-group minerals, Mg₃Si₂O₅(OH)₄, with possible magnetite, chromite, or brucite
AI-generated identification · may be incorrectDark green to nearly black, mottled and waxy to vitreous, with polished reflective surfaces. Typically Mohs 2.5–4, massive rather than visibly crystalline, and relatively dense (about 2.5–2.7); the rounded shape indicates tumbling or water wear.
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Physical properties
Dark green to nearly black, mottled and waxy to vitreous, with polished reflective surfaces. Typically Mohs 2.5–4, massive rather than visibly crystalline, and relatively dense (about 2.5–2.7); the rounded shape indicates tumbling or water wear.
Formation & geological history
Forms when ultramafic mantle rocks such as peridotite are hydrated during hydrothermal alteration, commonly in oceanic crust and tectonic mélange zones. The specimen is likely from a much older serpentinized rock body, not assignable to a precise age from the photograph.
Uses & applications
Used as a decorative stone, lapidary material, carvings, and occasionally architectural stone. Avoid prolonged skin contact with dust or cutting debris because some serpentinite bodies can contain asbestos-forming minerals.
Geological facts
Its green-black mottling and slick/waxy polish are characteristic of serpentine. It can resemble nephrite jade, but serpentine is generally softer and less tough.
Field identification & locations
Common in serpentinite belts of the Pacific Northwest, including areas around the North Cascades and Coast Range near 48.5°N, 122.3°W. Check for low hardness: a steel nail may scratch it; it usually will not scratch glass reliably. Identification is provisional without testing.
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