Jaspers

Jasper is an opaque variety of chalcedony, which is a microcrystalline variety of the mineral Quartz. Opaque means that neither light nor images pass through. Microcrystalline quartz in its pure form is semitransparent. When a small amount of impurities or foreign materials are added to microcrystalline quartz, the color changes and its ability to transmit light decreases. Jasper contains enough impurities and foreign material to make it opaque.

It often contains an abundance of impurities, and therefore some regard it as a rock instead of a mineral. Jasper is usually associated with brown, yellow, or reddish colors, but may be used to describe other opaque colors of Chalcedony such as dark or mottled green and orange. Some forms of Jasper are banded, and these banded Jaspers may appear similar to Agate, but unlike Agate they are opaque. When Jasper is dull and lacking interesting colors or patterns, it is not Jasper but rather Chert.

Jasper is an opaque rock of virtually any color stemming from the mineral content of the original sediments or ash. Patterns arise during the consolidation process forming flow and depositional patterns in the original silica rich sediment or volcanic ash. Hydrothermal circulation is generally thought to be required in the formation of jasper. The classification and naming of jasper varieties are attributed to various well-defined materials includes the geographic locality where it is found, sometimes quite restricted such as "Bruneau" (a canyon) and "Lahontan" (a lake), rivers and even individual mountains; many are fanciful, such as "forest fire" or "rainbow", while others are descriptive, such as "autumn" or "porcelain". A few are designated by the place of origin such as a brown Egyptian or red African jaspers.

Picture jaspers exhibit combinations of patterns such as banding from flow or depositional patterns, dendritic or color variations resulting in what appear to be scenes or images on a cut section. Diffusion from a center produces a distinctive orbicular appearance, i.e., leopard skin jasper, or linear banding from a fracture as seen in leisegang jasper. Healed, fragmented rock produces brecciated (broken) jasper.

While "picture jaspers" can be found all over the world, specific colors or patterns are unique to the geographic region from which they originate. For example Oregon’s Deschutes or Biggs jasper and Idaho's Bruneau jasper from the Bruneau River canyon are particularly fine examples. Other examples can be seen at Llanddwyn Island in Wales. A remarkable, pale, blue-green jasper, with patterns somewhat resembling those of malachite, occurs in a deposit at Ettutkan Mountain, Russia.