Petrified Wood

Hardness: 6.5-7 on Mohs scale (due to quartz replacement); Color: Varies widely, including shades of brown, gray, red, orange, black, and blue, depending on trace minerals present during silicification; Luster: Dull to waxy; Crystal structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None; Specific Gravity: 2.6-2.7

Formed when wood is buried under sediment or volcanic ash and protected from decay. Groundwater rich in dissolved silica flows through the sediment, replacing the original organic material of the wood cell by cell with silica (chalcedony, opal, or quartz). This process, known as permineralization and replacement, faithfully preserves the original structure of the wood, sometimes down to microscopic detail. Geological age varies greatly, from hundreds of thousands to hundreds of millions of years, depending on the age of the original wood and the time required for silicification.

Used extensively as a decorative stone, for carving, and in jewelry (cabochons, beads). Larger pieces are prized as landscape accents and collectible specimens. Its unique patterns and colors make it highly sought after by collectors.

Trace elements present during silicification influence the color of petrified wood: iron oxides/hydroxides often yield red, brown, and yellow hues; copper leads to blue/green; manganese to pink/orange; and carbon to black. The most famous example is the Petrified Forest National Park in Arizona, USA, which contains vast quantities of petrified wood from the Triassic period (around 225 million years ago).

Identify it in the field by its woody texture (bark, growth rings, cellular structure) preserved in stone. It will feel heavy for its size (due to mineralization) and will not burn or rot like regular wood. It often has a waxy or dull luster, and its hardness means it won't be easily scratched by a steel knife. Common locations include areas with past volcanic activity or sedimentary basins where ancient forests were covered rapidly, such as the southwestern United States (Arizona, Utah), Madagascar, and parts of Argentina and China. Collectors look for pieces with well-preserved detail, vibrant colors, and good polish potential.