igneous
Tumbled basalt (likely iron-stained basalt)
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-black, fine-grained, opaque, and weakly vitreous to dull; the rounded surface is polished by tumbling, with brown-orange iron-oxide staining. Hardness about 5–6 Mohs; no visible cleavage or crystals; specific gravity roughly 2.8–3.0.
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Physical properties
Dark gray-black, fine-grained, opaque, and weakly vitreous to dull; the rounded surface is polished by tumbling, with brown-orange iron-oxide staining. Hardness about 5–6 Mohs; no visible cleavage or crystals; specific gravity roughly 2.8–3.0.
Formation & geological history
Basalt is a rapidly cooled mafic volcanic rock formed from low-viscosity lava, commonly at oceanic volcanoes, mid-ocean ridges, and continental volcanic fields. Its exact age cannot be determined from the photograph; the specimen was subsequently mechanically tumbled and polished.
Uses & applications
Used as aggregate, paving and construction stone, landscaping material, and occasionally as an inexpensive decorative or cabochon-like specimen. Tumbled basalt has modest collector value.
Geological facts
The rusty brown areas are likely weathered iron-bearing minerals rather than a separate gemstone mineral. Basalt’s fine texture reflects rapid cooling at or near Earth’s surface.
Field identification & locations
Identify it by its substantial weight, opaque black color, fine texture, and lack of obvious crystals; a streak, magnetism test, or fresh fracture would help confirm it. Common worldwide, especially in volcanic regions and river gravels.
