igneous
Porous quartz-rich rock, likely weathered rhyolite or silicified volcanic breccia
Rhyolite or volcanic breccia; predominantly microcrystalline quartz (SiO₂) with altered feldspar and iron oxides
AI-generated identification · may be incorrectLight tan, cream, and rusty brown; rough, highly porous texture with a pale translucent quartz/chalcedony patch and earthy/vitreous luster. Quartz has Mohs hardness 7, no cleavage, and specific gravity about 2.6; the rock’s cavities and alteration lower its overall density.
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Physical properties
Light tan, cream, and rusty brown; rough, highly porous texture with a pale translucent quartz/chalcedony patch and earthy/vitreous luster. Quartz has Mohs hardness 7, no cleavage, and specific gravity about 2.6; the rock’s cavities and alteration lower its overall density.
Formation & geological history
Likely formed from silica-rich volcanic lava or fragmental volcanic deposits, later altered and weathered with iron-oxide staining. Exact geologic age cannot be determined from the photograph; such rocks range from Paleozoic to Cenozoic in many volcanic regions.
Uses & applications
Usually a field or collector specimen; similar silicified volcanic rocks may be used as decorative stone, aggregate, or lapidary material if the silica zones polish well.
Geological facts
The translucent patch may be chalcedony or quartz filling a former gas cavity or fracture. Iron oxides produce the rusty coloration and commonly indicate weathering of iron-bearing minerals.
Field identification & locations
Look for a very hard, glassy/translucent silica patch that scratches glass, irregular volcanic fragments or cavities, and rusty staining; acid testing is generally nonreactive. Common in eroded volcanic terrains worldwide.
