metamorphic
Serpentinite (likely)
Serpentinite, dominated by serpentine-group minerals (antigorite, lizardite, chrysotile), commonly with magnetite and minor chlorite/talc
AI-generated identification · may be incorrectDark gray-green, waxy to silky luster, fine-grained and massive with fibrous or mottled textures; Mohs hardness about 2.5–4, generally low density, and no visible cleavage. The pale metallic-looking flecks may be magnetite or talc-bearing alteration.
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Physical properties
Dark gray-green, waxy to silky luster, fine-grained and massive with fibrous or mottled textures; Mohs hardness about 2.5–4, generally low density, and no visible cleavage. The pale metallic-looking flecks may be magnetite or talc-bearing alteration.
Formation & geological history
Forms when ultramafic mantle rocks such as peridotite hydrate during low-temperature metamorphism and hydrothermal alteration, commonly in oceanic crust and subduction-zone mélanges. Ages range from modern alteration to Precambrian examples.
Uses & applications
Used as decorative stone, carvings, countertops, and aggregate, though asbestos-bearing varieties require caution. Collectible value is usually modest unless the specimen has vivid color, attractive patterning, or unusual associated minerals.
Geological facts
Serpentinite is California’s state rock and commonly hosts chromite, magnetite, nickel, and platinum-group minerals. Some varieties contain fibrous chrysotile asbestos.
Field identification & locations
Identify it by its green-gray color, smooth or waxy surface, low hardness, and commonly slippery feel; a steel knife may scratch it. Common localities include California, the Appalachians, New Zealand, Italy, and the Swiss Alps.
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