Serpentinite

Hardness: 2.5-4 (Mohs). Color: Green (shades from light to dark), often mottled or variegated, can also be blackish-green or yellowish-green. Luster: Waxy to greasy, silky (fibrous varieties) or dull. Crystal Structure: Not a single crystal structure as it's a rock; serpentine minerals within are typically monoclinic. It often exhibits a characteristic greasy or slick feel. Cleavage: Indistinct in massive forms, but can have foliated or scaly textures. Specific Gravity: 2.5-2.6.

Formed through the metamorphism of ultramafic rocks (like peridotite or dunite) during a process called serpentinization. This occurs when olivine and pyroxene minerals in these rocks react with water at low to moderate temperatures (around 100-500 °C) and pressures. This process often takes place in oceanic crust or subduction zones, where seawater infiltrates the rock. Geological Age: Varies widely depending on the age of the protolith and the metamorphic event, ranging from Precambrian to Cenozoic.

Construction: Used as a decorative dimension stone for cladding, flooring, and countertops due to its attractive green color and ability to take a polish. It's often incorrectly marketed as "green marble." Sculpture and Carving: Its relative softness makes it suitable for carving. Jewelry: Some attractive varieties are cut and polished into cabochons or beads. Source of Asbestos: Historically, some serpentinites contained chrysotile asbestos, which was mined for its fire-resistant properties. However, due to health risks, this use has largely ceased. Serpentine minerals also have potential for carbon sequestration.

The name 'serpentine' comes from the mottled green color and scaly appearance of the rock, which resembles a snake's skin. Serpentinite is the state rock of California. It can form unique ecosystems on its soils (serpentine barrens) because the soils derived from serpentinite are often poor in calcium and rich in heavy metals, leading to specialized plant communities. It's a key rock in understanding plate tectonics, particularly the process of hydration in subduction zones.

Identify it in the field by its characteristic green color (often variegated), waxy or greasy luster, and sometimes a smooth, slick feel. It's relatively soft and can be scratched with a knife. Look for fibrous textures in some varieties. Common locations: Found in mountain belts and ophiolite complexes worldwide, where oceanic crust or upper mantle rocks have been exposed and metamorphosed. Significant occurrences in the United States include California, Appalachian Mountains, and Oregon. Tips for collectors: Look for samples with interesting patterns, colors, or textures. Be aware that some serpentinites may contain trace amounts of asbestos (chrysotile), so handling fibrous varieties should be done with caution. Always perform a scratch test to confirm its relative softness compared to other green rocks.