metamorphic

Greenstone (likely altered basalt)

Metabasalt/greenschist; chiefly chlorite, actinolite, epidote, albite, and quartz

AI-generated identification · may be incorrect

Typically green-gray, fine- to medium-grained, dull to weakly silky, with hardness about 5–6 and no obvious cleavage; the photographed piece appears weathered and embedded in a coarse aggregate surface.

Identified September 4, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when basaltic lava was metamorphosed under low-grade, water-rich greenschist-facies conditions, commonly during ancient ocean-floor alteration or mountain building; age cannot be determined from the image.

Uses & applications

Used locally as crushed aggregate, road base, riprap, and occasionally as a decorative or carving stone; attractive polished pieces have minor collector value.

Geological facts

“Greenstone” is a field term rather than one exact mineral or rock; its green color generally comes from chlorite, epidote, or amphibole replacing original basalt minerals.

Field identification & locations

Identify it by its massive green-gray appearance, fine texture, and relatively dense, tough character; common in metamorphic belts worldwide, including the Appalachians, New England, western North America, and oceanic terranes.