Petrified Wood

Physical Properties

Hardness: 6.5–7 on Mohs scale. Color: Brown, black, or grey (due to organic carbon or iron). Luster: Dull to waxy. Crystal structure: Cryptocrystalline (chalcedony/quartz). Cleavage: None, conchoidal fracture.

Formation & Geological History

Formed through permineralization, where organic wood material is replaced by minerals (usually silica) in a low-oxygen environment, often following volcanic eruptions or burial by sediment. These specimens can range from the Devonian to the Holocene periods.

Uses & Applications

Used for decorative ornaments, jewelry (cabochons), furniture (slices for tabletops), and as geological teaching specimens.

Geological Facts

Petrified wood is the state gem of Washington and the state fossil of Arizona and Louisiana. It preserves the cellular structure of the original tree, allowing paleobotanists to identify the species of the ancient tree.

Field Identification & Locations

Identify in the field by looking for wood-grain textures, bark-like exteriors, or annual growth rings in a rock that is heavy and hard enough to scratch glass. Commonly found in areas like the Petrified Forest National Park in Arizona.

Identified on: 4/20/2026

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