Texas designated petrified palmwood as the official state "stone" in 1969. Petrified wood is actually a fossil (see list of all state fossils).

What is now arid Texas was a lush tropical forest 100 million years ago. Trees that fell into mineral-rich mud before having a chance to decay became petrified wood, which is actually a quartz-like stone. The organic wood cells were replaced over time by minerals, often retaining the detailed shape of the original prehistoric wood. Petrified wood is called the most beautiful of fossils (Washington's state gem is petrified wood).

The spotted look of palmwood is caused by fossilized rod-like structures within the original wood. Depending upon the angle the stone is cut, they show up as spots, tapering rods, or lines. Petrified palm wood is very hard and takes a wonderful polish, making beautiful jewelry. Petrified wood is also the state gem of Washington, and the state fossil of North Dakota and Louisiana.

Source:

Links:

Geology: Windows to the Universe - NASA
Mineral Matters: Kids Habitat - San Diego Natural Hist. Museum
Rocks and Rock Collections: Madison Falls HS Earth Science
Petrified Palm Wood: State Fossils.com
Petrified Wood by Your Gemologist Robert James
Petrified Wood by Paul V. Heinrich
The Mineral Calcite: Minerals by Name
Hell's Crust - Our Everchanging Planet: Savage Earth
State Fossils & Dinosaurs - 50 States