Jasper is an opaque, impure variety of silica, known for its diverse colors and patterns. It forms from microgranular quartz or cryptocrystalline chalcedony, making it a member of the broader quartz family. The term “jasper” derives from ancient words meaning “spotted stone,” aptly describing its variegated appearance. Jasper typically exhibits a hardness between 6.5 and 7 on the Mohs scale.
Geological Conditions for Jasper Formation
Jasper forms through geological processes involving the accumulation and alteration of silica-rich materials. Most commonly, it originates in sedimentary environments where silica-rich fluids permeate porous rocks or sediments. These fluids deposit microscopic quartz crystals, which gradually fill voids and replace existing material.
Silica content for jasper formation can derive from volcanic ash, marine organisms, or groundwater rich in dissolved minerals. Hydrothermal circulation, involving hot, silica-laden fluids, plays a role in this process, allowing for the slow precipitation of silica.
Diverse coloration and intricate patterns in jasper result from mineral impurities during formation. Iron oxides impart red, yellow, or brown hues, while chlorite creates green varieties. Manganese may create spotted patterns, and clay minerals contribute to white or gray tones.
While most commercial jasper deposits are sedimentary, the stone also forms in igneous and metamorphic settings. In igneous environments, silica-rich hydrothermal fluids can silicify volcanic rocks like rhyolitic flows, transforming them into jasper. Metamorphic processes, involving heat and pressure, can recrystallize microcrystalline quartz into jasper, as seen in jasper-bearing schists.
Key Global Deposits of Jasper
Jasper deposits are found globally, with specific regions producing unique varieties.
- United States: Oregon (Picture, Biggs, Owyhee Jaspers, displaying scenic landscape patterns); Idaho (Bruneau Jasper, known for its orbicular forms); California (Poppy Jasper, with vibrant red and yellow orb-like inclusions).
- Australia: Western Australia is known for Mookaite Jasper (striking reds, yellows, and purples) and Noreena Jasper.
- South Africa: Mines diverse jaspers like Leopard Skin Jasper and Elephant Skin Jasper.
- Madagascar: Famous for Ocean Jasper, found along its northwestern coast, distinguished by orbicular patterns and a wide color range.
- Mexico: Contributes Porcelain Jasper, Imperial Jasper (displaying fine, intricate designs), and Dalmatian Jasper from Chihuahua.
- Indonesia: A source of various jaspers from the Purbalingga district.
- Russia: Significant deposits in the Urals and Siberia yield colored jaspers, including landscape patterns.
- India: Locations in Rajasthan, Maharashtra, and Madhya Pradesh are known for diverse jasper forms.
- Brazil: Offers various jasper types, including red and green varieties.