Petrified wood (Araucarioxylon arizonicum) was designated the state fossil of Arizona in 1988. Petrified Forest National Park in northern Arizona has one of the world's largest and most colorful concentrations of petrified wood. Made up of almost solid quartz, each piece of petrified wood is like a giant crystal, often sparkling in the sunlight with a rainbow of colors.

Petrified wood log - photo from free public domain photo.org.
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Over 200 million years ago, the original logs were washed into an ancient river system and buried quickly by sediment so that oxygen was cut off and decay was slowed to a process that would take centuries. Minerals such as silica were absorbed into the porous wood over hundreds and thousands of years and crystallized within the cellular structure, replacing the organic material (petrified wood weighs as much as 150-200 pounds per cubic foot).