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Chalcedony in a Thunder Egg

Quartz and Silica Minerals Gallery

The core of this thunder egg is composed of chalcedony (kal-SED-a-nee), the uncrystallized form of silica. This is about as clear as chalcedony gets.
A chalcedony core
Photo (c) 2003 Andrew Alden, licensed to About.com (fair use policy)

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