igneous
Tumbled dark green mafic rock, likely basalt or diabase
Fine-grained basalt/diabase; chiefly plagioclase feldspar and pyroxene, with possible olivine and magnetite
AI-generated identification · may be incorrectTypically dark gray-green to black, fine-grained, massive, and dull to slightly vitreous; Mohs hardness about 5–6, no visible cleavage, and specific gravity roughly 2.8–3.1. The rounded shape and smooth surface indicate natural water wear or tumbling.
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Physical properties
Typically dark gray-green to black, fine-grained, massive, and dull to slightly vitreous; Mohs hardness about 5–6, no visible cleavage, and specific gravity roughly 2.8–3.1. The rounded shape and smooth surface indicate natural water wear or tumbling.
Formation & geological history
Forms when mafic magma cools, either rapidly as volcanic basalt or more slowly in shallow intrusions as diabase/dolerite; commonly Mesozoic to Recent, depending on locality. The green tint may reflect chlorite or other alteration minerals.
Uses & applications
Used as crushed aggregate, road stone, riprap, and landscaping material; attractive polished pieces are sold as inexpensive specimens or cabochons.
Geological facts
Its fine texture and dark color indicate iron- and magnesium-rich magma. Magnetite may make it weakly magnetic, while vesicles would suggest basalt rather than dense diabase.
Field identification & locations
Identify by its uniform dark fine grain, substantial weight, lack of layering, and possible magnetism; common in volcanic regions, oceanic crust, and glacial or river gravels. Exact identification requires a fresh surface or thin section.
