Serpentinite

Hardness: 2.5-4 (Mohs Scale); Color: Mottled green, dark green, bluish-green, greenish-black; Luster: Greasy, waxy, silky; Crystal Structure: Cryptocrystalline to fine-grained aggregates of serpentine minerals (e.g., antigorite, chrysotile, lizardite); Cleavage: Perfect to none, depending on the constituent minerals; Specific Gravity: 2.5-2.6

Serpentinite forms through the metamorphism of ultramafic rocks (like peridotite and dunite) in the presence of water, a process called serpentinization. This typically occurs in subduction zones or mid-ocean ridge environments. The geological age varies widely depending on the original ultramafic rock and the timing of metamorphic events, ranging from Precambrian to Cenozoic.

Serpentinite is used as an ornamental stone in architecture for facing buildings, as a decorative carving material, and occasionally as a gemstone (often called 'noble serpentine' or 'verde antique'). Its resistance to heat and chemicals makes some varieties historically useful as asbestos (chrysotile serpentine), though its use is now heavily restricted due to health concerns. It can also be a source of magnesium.

Serpentinite gets its name from its characteristic snake-like (serpentine) mottled green appearance. The process of serpentinization releases hydrogen gas, which can support chemosynthetic life in deep-sea environments. The variety 'verde antique' is a prized ornamental stone. Serpentinite rocks sometimes host ore deposits of nickel, chromium, and cobalt.

In the field, serpentinite can be identified by its distinctive greasy or waxy luster, its mottled green color, and its smooth, sometimes slippery feel. It often occurs in areas of intense tectonic activity, particularly along fault zones and mountain belts where oceanic crust has been subducted or obducted. Common locations include the Appalachian Mountains (eastern North America), California (Coast Ranges), the Alps, and various ophiolite complexes worldwide. Collectors appreciate its varied green hues and suitability for carving.