mineral
Tumbled magnetite (likely)
Magnetite, Fe₃O₄; possibly magnetite-rich ironstone
AI-generated identification · may be incorrectBlack to dark gray, metallic-to-submetallic luster, opaque, commonly massive rather than visibly crystalline; Mohs hardness 5.5–6.5, cubic crystal system, no distinct cleavage, specific gravity about 5.2. The smooth surface appears polished or naturally water-worn.
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Physical properties
Black to dark gray, metallic-to-submetallic luster, opaque, commonly massive rather than visibly crystalline; Mohs hardness 5.5–6.5, cubic crystal system, no distinct cleavage, specific gravity about 5.2. The smooth surface appears polished or naturally water-worn.
Formation & geological history
Forms when iron-rich magma crystallizes, during metamorphism of iron formations, or by hydrothermal alteration; magnetite deposits range from Precambrian banded iron formations to younger igneous and sedimentary settings.
Uses & applications
Used chiefly as an iron ore, dense aggregate, pigment, and magnetic material; attractive polished pieces are sold as specimens or curiosities.
Geological facts
Magnetite is strongly magnetic and is the principal naturally magnetic mineral; its name derives from the ancient Magnesia region. This specimen’s identity should be confirmed with a strong magnet and streak test.
Field identification & locations
Common in igneous rocks, black sands, skarns, and banded iron formations worldwide, especially Sweden, Australia, South Africa, the United States, and Brazil. A black streak, high heft, and strong attraction to a magnet distinguish it from obsidian or hematite.
