igneous
Weathered basalt or andesitic volcanic rock
Porphyritic basalt–andesite; likely plagioclase and pyroxene with iron-oxide alteration
AI-generated identification · may be incorrectDark gray to brown, fine-grained and locally porphyritic, with dull to slightly vitreous luster and no obvious cleavage; approximate Mohs hardness 5–6, specific gravity 2.7–3.0. Orange-brown areas are likely limonite/goethite weathering products.
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Physical properties
Dark gray to brown, fine-grained and locally porphyritic, with dull to slightly vitreous luster and no obvious cleavage; approximate Mohs hardness 5–6, specific gravity 2.7–3.0. Orange-brown areas are likely limonite/goethite weathering products.
Formation & geological history
Formed when mafic to intermediate lava cooled at or near Earth’s surface, probably millions to billions of years ago; subsequent weathering oxidized iron-bearing minerals. The rounded shape is consistent with stream or beach abrasion.
Uses & applications
Used as crushed aggregate, road stone, riprap, and occasionally architectural stone; this specimen is mainly suitable for educational or decorative collecting.
Geological facts
The mottled orange-black appearance reflects oxidation along fractures and vesicles rather than a separate gemstone mineral. Exact identification requires a fresh surface, hand lens, streak, and possibly thin-section or X-ray analysis.
Field identification & locations
Common in volcanic terrains and transported river gravels; identify by its dense dark matrix, feldspar crystals, lack of strong layering, and magnetic response if magnetite is present. Estimated value: $2–$15 as a common hand specimen.
