sedimentary

Petrified Wood

Silicified Wood (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Dark brown, black, with visible wood grain; Luster: Dull to greasy; Crystal Structure: Cryptocrystalline; Cleavage: None; Specific Gravity: 2.5-2.8.

Hardness: 6
Color: Dark brown, black, with visible wood grain
Luster: Dull to greasy

Identified May 20, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 (Mohs scale); Color: Dark brown, black, with visible wood grain; Luster: Dull to greasy; Crystal Structure: Cryptocrystalline; Cleavage: None; Specific Gravity: 2.5-2.8.

Formation & geological history

Formed via permineralization where organic wood material is replaced by silica (usually chalcedony or quartz) over millions of years by being buried in sediment and saturated by groundwater. Often found in deposits from the Triassic to the Cretaceous periods.

Uses & applications

Used for decorative items, lapidary work (jewelry and cabochons), architectural accents, and as a popular focus for geological collectors.

Geological facts

Petrified wood is the state fossil of Arizona, North Dakota, and Louisiana. Despite being made of stone, it often preserves the microscopic structure of the original wood perfectly, including tree rings.

Field identification & locations

Identify in the field by looking for distinct wood-like bark texture or internal growth rings on an exceptionally heavy, hard stony specimen. Common in arid sedimentary basins like the Petrified Forest National Park region.