Petrified wood is a type of fossil: it consists of fossil wood where all the organic materials have been replaced with minerals (most often a silicate, such as quartz), while retaining the original structure of the wood. The petrifaction process occurs underground, when wood becomes buried under sediment. Mineral-rich water flowing through the sediment deposits minerals in the plant's cells and as the plant's lignin and cellulose decay away, a stone mold forms in its place. The wood is preserved due to a lack of oxygen.

Minerals such as manganese, iron, and copper in the water/mud during the petrification process give petrified wood a variety of color ranges. Quartz crystals are colorless, but when iron is added to the process the crystals become stained with a yellow or red tint.

Following is a list of minerals and related color hues:

• carbon - black
• cobalt - green/blue
• chromium - green/blue
• copper - green/blue
• iron oxides - red, brown, yellow
• manganese - pink/orange
• manganese oxides - black
• silica - white, grey

Petrified wood can preserve the original structure of the wood in all its detail, down to the microsopic level. Structures such as tree rings and the various tissues are often observed features.

Petrified wood has a Mohs hardness level of 7, the same as quartz, materials from which scientists have been able to make artificial petrified wood recently [1].

Petrified wood is also the state gem of Washington.

from Wikipedia