Rhyolite is a group of extrusive igneous rocks, rocks that formed from the solidification of molten rock material. Thick granitic lava that forms Rhyolite often cools quickly while pockets of gas are still trapped inside of the lava. The trapped gas is unable to escape, forming cavities known as "vugs."  
 
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* Photos show both sides.
* Sizes, shapes and colors my vary due to being a natural stone, being hand-carved and/or difference in computer monitors.