igneous
Mixed rounded river pebbles, likely basalt and granite
Basalt (fine-grained mafic rock) and granite/granitoid (quartz–feldspar rock)
AI-generated identification · may be incorrectThe smooth dark gray pebble is likely basalt: Mohs 5–6, dull to slightly glassy, fine-grained, dense, and lacking visible cleavage. The lighter speckled pebbles are probably granitic: Mohs about 6–7, granular texture, quartz and feldspar crystals, with minor mica; exact identification requires close examination.
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Physical properties
The smooth dark gray pebble is likely basalt: Mohs 5–6, dull to slightly glassy, fine-grained, dense, and lacking visible cleavage. The lighter speckled pebbles are probably granitic: Mohs about 6–7, granular texture, quartz and feldspar crystals, with minor mica; exact identification requires close examination.
Formation & geological history
These are weathered and water-rounded fragments eroded from exposed bedrock, commonly Quaternary or older source rocks. Basalt forms from cooled lava, while granite crystallizes slowly from magma deep underground.
Uses & applications
Used as landscaping stone, drainage aggregate, concrete aggregate, and decorative gravel; these small pieces have minimal commercial value but are suitable for casual collecting.
Geological facts
Rounded edges and surface smoothing indicate transport by a stream, river, or shoreline rather than freshly broken bedrock. The pale stones may also include weathered metamorphic rock, so the group is not a single specimen.
Field identification & locations
Identify basalt by its uniform dark color and very fine grain; identify granite by visible contrasting quartz and feldspar grains. Common worldwide in streambeds and glacial deposits.
