Red beryl on rhyolite matrix
Designated the world’s rarest coloured gemstone in 2006 by the Jewellers association, red beryl only occurs in three locations in the USA, with the only decent specimens coming from the now moribund Ruby Violet mine in Utah. The matrix it grows in is a particular kind of silica rich magma formed during either crustal melting (say in a mountain building event) or when magmas such as basalt gradually differentiate during cooling. As crystals of olivine, pyroxene and plagioclase form and settle in the magma chamber, the remaining lava becomes enriched in silica and elements that do not fit into the structure of these minerals (known as incompatible elements and extensively used in geochemistry).
Beryl is of course the same mineral that gives us lovely blue aquamarines or yellow heliodor (when coloured by Iron), green emerald (chromium) and pink morganite (manganese). It usually forms in pegmatites, the last remains of granites to slowly crystallise, producing large crystals sometimes filled with rare elements distilled out of the planet by volcanism. In this case though, the beryl formed directly from a gaseous state, when a younger rhyolite intruded beneath the one in which it formed, and the gasses flowed up as it cooled along the joints produced in a manner similar to the Giant’s Causeway in Ireland.
Another post on red beryl: http://tinyurl.com/olaegwd
Image credit: Eric Hunt