Greenstone

Physical Properties

Hardness: 5-7 Mohs; Color: Dull grayish-green with reddish-brown oxidation/limonite staining; Luster: Earthy to slightly waxy; Structure: Fine-grained, massive; Cleavage: None (conchoidal to irregular fracture); Specific Gravity: 2.85-3.0.

Formation & Geological History

Formed through low-grade metamorphism (greenschist facies) of mafic volcanic rocks like basalt or gabbro. This process typically occurs at subduction zones or mid-ocean ridges where heat and chemical fluids alter the original minerals into green-tinted minerals like chlorite, epidote, and actinolite.

Uses & Applications

Historically used for making stone tools (celts and axes); modernly used for crushed stone in construction, road base, and occasionally as architectural decorative stone or for landscape design.

Geological Facts

Greenstones are often found in very ancient geological regions known as 'Greenstone Belts,' which dates back to the Archean and Proterozoic eons, some of the oldest crustal fragments on Earth.

Field Identification & Locations

Identify in the field by its distinct dull green hue (revealed when chipped or wet) and its relative hardness compared to sedimentary rocks. It often has a rusty weathered exterior due to iron content. Commonly found in mountainous regions or ancient shield areas like the Appalachians or the Canadian Shield.

Identified on: 4/26/2026

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