igneous
Dalmatian jasper (likely orbicular rhyolite)
Silica-rich rhyolite, commonly sold as Dalmatian jasper; typically quartz, feldspar, and dark amphibole/biotite inclusions
AI-generated identification · may be incorrectHardness about 6–7 Mohs; pale gray to cream matrix with black-green spots, opaque, fine-grained, and generally matte to waxy when polished. No visible cleavage; specific gravity typically about 2.5–2.7.
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Physical properties
Hardness about 6–7 Mohs; pale gray to cream matrix with black-green spots, opaque, fine-grained, and generally matte to waxy when polished. No visible cleavage; specific gravity typically about 2.5–2.7.
Formation & geological history
Forms from rapidly cooled silica-rich volcanic lava, with contrasting dark mineral inclusions producing the spotted pattern; specimens marketed as Dalmatian jasper are commonly associated with Mexican rhyolite, generally Cenozoic in age.
Uses & applications
Used for cabochons, tumbled stones, beads, carvings, and decorative objects; modest jewelry and lapidary value.
Geological facts
Despite its trade name, it is usually not true jasper—a sedimentary microcrystalline quartz variety—but an igneous rock. The spots are mineral inclusions rather than fossil markings.
Field identification & locations
Identify by its cream-gray fine-grained body, irregular black spots, and hardness sufficient to scratch glass; test with a streak plate or magnification to distinguish it from dyed stone. Common sources include Mexico and parts of the western United States.
