igneous
Agate in Rhyolite
Silicon Dioxide (Quartz/Chalcedony) in Volcanic Rock matrix
Hardness of 7 on the Mohs scale for the quartz/chalcedony portion; reddish-brown, pink, and white coloration; vitreous to waxy luster; cryptocrystalline structure; specific gravity around 2.65
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Physical properties
Hardness of 7 on the Mohs scale for the quartz/chalcedony portion; reddish-brown, pink, and white coloration; vitreous to waxy luster; cryptocrystalline structure; specific gravity around 2.65
Formation & geological history
Formed within gas cavities (vesicles) in cooling volcanic lava flows, specifically rhyolite, where silica-rich hydrothermal solutions deposited microcrystalline quartz layers over millions of years
Uses & applications
Used in lapidary arts, mineral collecting, making decorative items, and sometimes cut into cabochons for jewelry
Geological facts
These specimens often form when silica-bearing groundwater percolates through cavities in ancient volcanic rocks, slowly precipitating out layers of chalcedony and quartz crystals
Field identification & locations
Easily identified by the stark contrast between the dull, reddish volcanic host rock (rhyolite) and the bright, crystalline or banded quartz/chalcedony core lining a pocket or vug
