Petrified wood (from the Greek root "petro" meaning "rock" or "stone", literally "wood turned into stone") 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 and is initially preserved due to a lack of oxygen. 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 mould forms in its place. In general, wood takes less than 100 years to petrify. The organic matter needs to become petrified before it decomposes completely. Elements such as manganese, iron and copper in the water/mud during the petrification process give petrified wood a variety of color ranges. Pure quartz crystals are colorless, but when contaminants are added to the process the crystals take on a yellow, red, or other tint.