Petrified Wood

Physical Properties

Hardness: 6.5–7 (Mohs scale); Color: Brown, tan, reddish-brown, and grey; Luster: Pearly to vitreous; Crystal structure: Microcrystalline (Cryptocrystalline) quartz; Cleavage: None; Specific Gravity: 2.6–2.9.

Formation & Geological History

Formed through permineralization where organic wood materials are replaced by minerals (usually silica) while retaining the original structure. This occurs when wood is buried under sediment or volcanic ash, excluding oxygen and preventing decay. Most specimens are from the Triassic, Jurassic, or Tertiary periods.

Uses & Applications

Used primarily for ornamental purposes, jewelry (cabochons), interior design (tabletops/bookends), and as educational museum or collector specimens.

Geological Facts

Famous locations include the Petrified Forest National Park in Arizona, USA. It is the state gem of Washington. The preservation can be so perfect that individual tree rings and even cellular structures can be seen under a microscope.

Field Identification & Locations

Identify in the field by looking for 'bark' textures and tree ring patterns on heavy, stone-like objects. It will feel much heavier and colder than real wood and cannot be scratched by a steel knife. Common in sedimentary basins with high volcanic ash content.

Identified on: 4/26/2026

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