metamorphic
Petrified Wood
Silicified Wood (SiO2)
Hardness: 6.5–7 on Mohs scale; Color: Variable (browns, tans, greys, and black); Luster: Vitreous to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.9.
- Hardness
- 6
- Color
- Variable (browns, tans, greys, and black)
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6.5–7 on Mohs scale; Color: Variable (browns, tans, greys, and black); Luster: Vitreous to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.9.
Formation & geological history
Formed via permineralization, where organic wood material is replaced by silicates—usually chalcedony or quartz—while buried under sediment or volcanic ash. This happens over millions of years (usually 20 to 300 million years old) in anaerobic environments.
Uses & applications
Used primarily in jewelry (cabochons), interior design (polished tabletops/bookends), and as a sought-after collectible for fossil and mineral enthusiasts.
Geological facts
Petrified wood is the state gem of Washington and Oregon. The most famous location is the Petrified Forest National Park in Arizona, which contains logs from the Late Triassic Period (~225 million years ago).
Field identification & locations
Identify in the field by looking for bark-like textures on the exterior and visible growth rings or wood cell structures on broken surfaces. It is much heavier and harder than normal wood and will not burn. Common in sedimentary basins and volcanic deposit areas.
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