Petrified Wood (Agatized/Silicified Wood)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Reddish-brown, tan, and gray due to iron oxides and manganese; Luster: Vitreous to waxy; Crystal structure: Microcrystalline (Cryptocrystalline) quartz; Cleavage: None/Conchoidal fracture; Specific gravity: 2.6-2.9.

Formation & Geological History

Formed through permineralization where organic plant matter is replaced by silica (usually chalcedony or opal) in an anaerobic environment, typically buried under volcanic ash or river sediments. Geological age often ranges from the Devonian to the Pleistocene (390 million to 10,000 years ago).

Uses & Applications

Commonly used in jewelry (cabochons), lapidary art, furniture (tabletops), decorative bookends, and as a popular teaching aid for paleontology and geology.

Geological Facts

Petrified wood is the state fossil of several U.S. states, including Arizona and Washington. The cell structure of the original tree is sometimes preserved so perfectly that the species of tree can be identified under a microscope.

Field Identification & Locations

Identify in the field by looking for wood-like grain, bark textures, or growth rings in a heavy, stone material that cannot be scratched by a steel knife. Common in the western United States, Madagascar, and Brazil.

Identified on: 4/13/2026

Mode: Standard