Rhyolite

Banded Rhyolite, also known as Wonderstone, is not a mineral but is a fine-grained, light-colored igneous, volcanic (extrusive) rock of felsic (silicon-rich) composition that is typically 70-75% SiO₂ (silicon dioxide). Rhyolite is chemically equivalent to granite. It is composed of the minerals Quartz and Orthoclase Feldspar with smaller amounts of Plagioclase Feldspar, Biotite Mica, Amphiboles, and Pyroxenes. Rhyolites that cool too quickly to grow crystals form a natural glass called Obsidian. Rhyolite often consists of wavey bands of light and dark in beautiful colors of tan, brown, orange and red and can be used to create unique cabochons and faceted gems.

Rhyolite is known from all parts of the Earth and from all geologic ages. It is one of the most common volcanic rocks in continental regions but is virtually absent in the ocean basins.