Greenschist (Greenstone)

Physical Properties

Hardness: 3.5 to 4.5; Color: Dull grayish-green to dark green; Luster: Pearly to dull; Crystal structure: Usually foliated or massive; Cleavage: Can exhibit schistosity; Specific gravity: 2.8 to 3.0. Often contains chlorite, epidote, and actinolite.

Formation & Geological History

Formed through low-grade regional metamorphism of mafic igneous rocks (such as basalt) or tuff under relatively low temperature and pressure conditions (approx. 300 to 450 degrees Celsius). This process primarily occurs at tectonic subduction zones or mountain-building regions.

Uses & Applications

Historically used for tools and weapons (celt axes). In modern times, it is used for decorative garden stone, construction aggregate, and occasionally by collectors or for lapidary work if the color is particularly vibrant.

Geological Facts

The characteristic green color of the rock comes from the presence of chlorite, actinolite, and epidote minerals. It is a key indicator of the 'greenschist facies' in metamorphic petrology, identifying specific conditions of the Earth's crustal history.

Field Identification & Locations

Identify in the field by its distinctive green-gray hue, fine-grained texture, and sometimes a slightly slippery feel if chlorite-rich. It is commonly found in the Appalachian Mountains, the Alps, and coastal ranges of California. Look for it in areas with high past tectonic activity.

Identified on: 4/13/2026

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