Rhyolite ~ Brown Mushroom Jasper
Rhyolite is a relatively rare volcanic rock that exists in natural colors ranging from moss green, light-to-dark greens, splashes of terracotta reds and oranges, and touches of white and brown. The stone begins in a molten state that cools too quickly for crystals to form. Because ryolitic magma and lava are so thick, they tend to build up pressure until a gas explosion in the volcano throws them out in particles ranging from ash all the way up to big blocky chunks. The decorative and ornamental stones that are cut and used for jewelry are formed when the lava flow "bands" and is literally frozen into the stone. Stones with more green are often called Rainforest Jasper because of their forest-like colorations, but they are actually an igneous rock with a high silica content — chemically identical to granite. (This website has a separate page for Green/Rainforest Ryolite/Jasper). Rainbow or Peacock Rhyolite are lighter in color than typical Brown and Green Rhyolite, and have more distinct areas of red banding. Mushroom Rhyolite gets its name from the small fungal-like shapes in gray to brown, which are usually bordered with red halos - and occasionally brecciated (solidified) with quartz against a green background. All varieties are found in the USA and Africa. An interesting tidbit: Although the terms Ryolite and Rhyolite are generally used interchangeably, geologists refer to "Rhyolite" (Mushroom Rhyolite) as being grey, brown, or having a reddish-terra cotta hue. Both varieties are found in the USA and Africa.