mineral

Quartz geode with chalcedony and iron staining

Silica geode; predominantly quartz (SiO₂) and microcrystalline quartz/chalcedony

AI-generated identification · may be incorrect

Hardness 7 for quartz, about 6.5–7 for chalcedony; vitreous to waxy luster, gray-white to reddish-brown banding, and drusy/crystalline cavities. Quartz is trigonal and typically lacks cleavage, with conchoidal fracture; visible cream crystals may include calcite, requiring testing.

Identified September 13, 2026More mineral →

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Physical properties

Formation & geological history

Likely formed when silica-rich groundwater filled vesicles or cavities in volcanic rock, depositing chalcedony first and quartz crystals later; iron oxides produced the red-brown colors. Exact age and host rock cannot be determined from the image.

Uses & applications

Used as a decorative and collector specimen; polished geodes and quartz are used in ornaments, jewelry, and lapidary work. Value depends strongly on crystal quality, size, and whether the cavity is attractive.

Geological facts

The concentric reddish-gray layers are characteristic of chalcedony or agate, while the cavity contains later mineral growth. Acid testing or hardness testing can help distinguish possible calcite from quartz.

Field identification & locations

Identify by banded silica rind, hollow interior, and sparkling quartz druse; common sources include Brazil, Uruguay, Mexico, Morocco, and the western United States. Avoid destructive tests on valuable specimens.