mineral
Quartz-lined agate/geode slice
Microcrystalline quartz (chalcedony, SiO₂) with drusy quartz; possibly calcite inclusions
AI-generated identification · may be incorrectGray-white, irregularly rounded polished slice with a fine crystalline/drusy sparkle and darker chalcedony rim; quartz hardness ~7 Mohs, vitreous to dull luster, trigonal crystal structure, no cleavage, specific gravity ~2.65. Exact mineral identification is limited by the photograph.
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Physical properties
Gray-white, irregularly rounded polished slice with a fine crystalline/drusy sparkle and darker chalcedony rim; quartz hardness ~7 Mohs, vitreous to dull luster, trigonal crystal structure, no cleavage, specific gravity ~2.65. Exact mineral identification is limited by the photograph.
Formation & geological history
Silica-rich groundwater filled a cavity in volcanic rock, depositing chalcedony and later quartz crystals; commonly Cenozoic to Mesozoic in age, depending on locality. The specimen has been cut and polished.
Uses & applications
Used as a decorative stone, paperweight, cabochon, or collector specimen; generally modest commercial value unless unusually attractive or locality-documented.
Geological facts
The sparkling surface is drusy quartz—many tiny quartz crystals coating a cavity wall. Agate is technically banded or patterned chalcedony, so this may be better described as a quartz geode or agate-like slice.
Field identification & locations
Look for a smooth chalcedony rind, concentric banding, cavity filling, and glassy quartz crystals; a hardness test, streak test, and acid test can distinguish quartz from calcite. Common sources include Brazil, Uruguay, Mexico, India, and the western United States.
