Rhyolite

Rhyolite is a type of igneous rock that's formed from volcanic magma and is rich in quartz and feldspar. Whilst a beautiful generally pale coloured rock in its own right, rhyolite is well known in mineral hunting circles as a good place to hunt for gems; rhyolite cavities (called 'vugs') within the rhyolite rock are sometimes lined with mineral crystals and gemstones. 

These vugs form through various geological processes and provide environments perfect for the growth of gemstones and minerals; one way that vugs form is that gas bubbles are sometimes trapped in the molten lava during volcanic eruptions; this lava cools and solidifies into rhyolite, and the gas bubbles then become cavities (vugs) in the rock. Later geological activity can then introduce mineral-rich fluids into these vugs, and it's these fluids that contribute to the growth of gemstones and minerals. The conditions within the vug, such as temperature, chemical elements, ions and mineral content will determine the type of gemstones that develop.