Serpentine

Hardness: 2.5-4 (Mohs), Color: Typically dark green, yellowish-green, brownish-green, or black. Can also be lighter green or mottled. Luster: Waxy to greasy, or dull. Crystal Structure: Monoclinic, but crystals are rare; usually massive, fibrous, or granular. Cleavage: None, but can have parting. Specific Gravity: 2.5-2.6.

Formed through the metamorphism of ultramafic rocks (like peridotite and dunite) by the hydration and alteration of olivine and pyroxene minerals. This process, called serpentinization, typically occurs at low-to-medium metamorphic grades, often in ocean spreading centers or subduction zones. Geological age varies depending on the regional geology.

Commonly used as an ornamental stone in architecture for facing and decorative items, known as 'verde antique' or 'serpentinite marble'. It's also used for carvings, sculptures, and occasionally as a gemstone (bowenite). Historically, some forms were used as asbestos. It can also be a source of magnesium.

Serpentine is the state rock of California. It can absorb carbon dioxide and is being researched for carbon sequestration. Asbestos is a form of serpentine minerals (chrysotile), which is fibrous and known for its heat-resistant properties, although its use has been largely discontinued due to health risks.

In the field, serpentine is identifiable by its typically dark green color, waxy to greasy luster, and often smooth, slippery feel. It can also have a mottled appearance with lighter or darker spots. It is relatively soft, so it can be scratched with a knife. Serpentine is commonly found in metamorphic terranes, especially in areas with ancient ocean crust or mantle rocks exposed at the surface. Look for it in ophiolite complexes, mountain ranges, and areas with extensive faulting. Tips for collectors: Be wary of dust if handling fibrous varieties, though the massive form is generally safe. It often polishes well.