Polished petrified wood; photo by Daniel Schwen on Wikipedia (use permitted with attribution / share alike).
Official State Gem of Washington
Washington designated petrified wood as the state gem in 1975. Though rock-hard and jewel-like when polished, petrified wood is actually a fossil, not a gemstone.
Petrified wood is also a state symbol of Texas and Mississippi (the state stone), and of Arizona, North Dakota and Louisiana (official state fossil). All State Gems - All State Fossils
Petrified Wood in Washington State
In prehistoric times, the interior of Washington was swampy with trees such as cypress, oak, elm and gingko. Some of these ancient forests were preserved by lava flows and water and over time became fossilized. You can see petrified wood at Gingko Petrified Forest State Park in Vantage, Washington.