Serpentinite (River Pebble)

Physical Properties

Hardness: 2.5 to 5 (Mohs); Color: Mottled green, black, grey, and reddish-brown; Luster: Waxy to greasy; Crystal Structure: Monoclinic/Orthorhombic (fibrous/lamellar); Cleavage: Poor; Specific Gravity: 2.5-3.3

Formation & Geological History

Formed through 'serpentinization,' a low-temperature metamorphic process where ultramafic rocks from the Earth's mantle are hydrated by hydrothermal fluids at tectonic plate boundaries. This specimen is water-worn into a pebble shape.

Uses & Applications

Source of magnesium and asbestos (historically); widely used as decorative stone, architectural facing, and for stone carvings. Quality pieces are used for costume jewelry beads.

Geological Facts

Serpentinite is the official state rock of California. It often creates unique soil chemistry (high in magnesium, low in calcium) that supports rare, endemic plant species.

Field Identification & Locations

Identify by its 'snakeskin' mottled appearance and waxy feel. Often found in ophiolite complexes and along fault zones. In the field, look for a smooth, slightly soapy texture compared to surrounding stones.

Identified on: 4/19/2026

Mode: Standard