Petrified Wood

Physical Properties

Hardness: 6.5–7 (Mohs scale); Color: Brown, tan, cream, and reddish hues; Luster: Vitreous to waxy; Structure: Cryptocrystalline (replacing organic cellular structure); Cleavage: None (conchoidal fracture).

Formation & Geological History

Formed through permineralization where organic wood materials are replaced by silica (usually chalcedony or quartz) by groundwater. This process occurs over millions of years, often in volcanic ash beds or sedimentary deposits where wood was buried rapidly in low-oxygen environments.

Uses & Applications

Used in jewelry (cabochons), lapidary arts, high-end furniture (slabs), decorative ornaments, and as educational geological specimens.

Geological Facts

Petrified wood can preserve the original cellular structure of the tree so perfectly that species can be identified under a microscope. Arizona's Petrified Forest National Park contains one of the world's largest concentrations of these fossils, dating back to the Triassic period (~225 million years ago).

Field Identification & Locations

Identify by looking for wood-like grain, growth rings, or bark textures on a stone that is much harder than wood (cannot be scratched by a steel knife). Commonly found in dry, eroded landscapes or volcanic basins like those in the American West, Madagascar, and Indonesia.

Identified on: 4/14/2026

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