metamorphic
Glacially rounded metamorphic boulder
Likely foliated amphibolite or gneiss; exact identification requires a fresh surface and mineral examination
AI-generated identification · may be incorrectDark green-gray, fine- to medium-grained, weakly banded/foliated, with dull to slightly satin luster and rounded glacial abrasion. Typical hardness is about 5–7 Mohs; composition may include amphibole, plagioclase, biotite, and quartz.
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Physical properties
Dark green-gray, fine- to medium-grained, weakly banded/foliated, with dull to slightly satin luster and rounded glacial abrasion. Typical hardness is about 5–7 Mohs; composition may include amphibole, plagioclase, biotite, and quartz.
Formation & geological history
Formed when basaltic or sedimentary parent rock underwent regional metamorphism, commonly during Precambrian to Paleozoic mountain-building. Its smooth shape likely results from glacial transport and weathering, rather than crystal growth.
Uses & applications
Primarily used as landscaping or construction aggregate; attractive pieces may be collected as field specimens, though this example has limited gem value.
Geological facts
Green coloration commonly reflects chlorite, amphibole, or epidote. The rusty streaks are likely iron-oxide weathering along fractures or foliation.
Field identification & locations
Look for aligned dark minerals or subtle bands, dense weight, and resistance to scratching with a steel knife; a fresh broken surface would distinguish amphibolite from gneiss or greenstone. Common in glacial regions, especially the northern United States, Canada, Scandinavia, and northern Europe.
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