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.