igneous
Orbicular rhyolite (orbicular jasper-like rhyolite)
Silica-rich rhyolite containing orbicules, typically concentric feldspar–quartz aggregates with iron-oxide staining
AI-generated identification · may be incorrectUsually gray, tan, pink, or brown with rounded concentric orbicules; fine-grained volcanic groundmass, vitreous to dull luster, no prominent cleavage, and hardness about 6–7 Mohs. Composition is chiefly quartz, alkali feldspar, plagioclase, and minor biotite or iron oxides.
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Physical properties
Usually gray, tan, pink, or brown with rounded concentric orbicules; fine-grained volcanic groundmass, vitreous to dull luster, no prominent cleavage, and hardness about 6–7 Mohs. Composition is chiefly quartz, alkali feldspar, plagioclase, and minor biotite or iron oxides.
Formation & geological history
Forms when silica-rich lava cools and crystallizes around nucleation centers, producing concentric orbicules; associated with volcanic caldera or dome environments. Most examples are Cenozoic, though exact age depends on locality.
Uses & applications
Used as a lapidary stone for cabochons, slabs, carvings, and ornamental objects; rarely used structurally because its pattern and relative scarcity favor collecting and jewelry.
Geological facts
Its orbicules may resemble eyes or flowers and can show alternating feldspar-, quartz-, and iron-rich bands. It is often marketed as orbicular jasper, although true jasper is sedimentary or hydrothermal silica rather than rhyolite.
Field identification & locations
Identify it by contrasting rounded, concentric orbicules embedded in a hard volcanic matrix; a fresh surface should show tiny interlocking crystals rather than glassy conchoidal fracture. Notable material occurs in Australia, Mexico, Madagascar, and parts of the western United States.
