metamorphic
Greenstone / Serpentinite
Antigorite and Lizardite mixture (Mg3Si2O5(OH)4)
Hardness: 3-6 Mohs. Color: dull green to grayish-black. Luster: waxy to greasy on broken surfaces. Crystal structure: Monoclinic (usually massive). Specific gravity: 2.5-3.0.
- Hardness
- 3-6 Mohs
- Color
- dull green to grayish-black
- Luster
- waxy to greasy on broken surfaces
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Physical properties
Hardness: 3-6 Mohs. Color: dull green to grayish-black. Luster: waxy to greasy on broken surfaces. Crystal structure: Monoclinic (usually massive). Specific gravity: 2.5-3.0.
Formation & geological history
Formed by the hydration of ultramafic rocks (like peridotite) from the Earth's mantle through a process called serpentinization, typically occurring at oceanic plate boundaries.
Uses & applications
Used historically for decorative sculptures, architectural stone, and as a source of magnesium. High-grade varieties are used in lapidary work.
Geological facts
Serpentinite is the state rock of California. It often contains traces of magnetite, making some specimens slightly magnetic. Small veins of chrysotile (asbestos) can sometimes be found within it.
Field identification & locations
Identify by its characteristic 'serpentine' green waxy appearance and slickensided (greasy) surfaces. Commonly found in ophiolite belts where tectonic plates collide.
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