PHOTOSTOCK-ISRAEL / SCIENCE PHOTO LIBRARY PHOTOSTOCK-ISRAEL / SCIENCE PHOTO LIBRARY
Crystal geode. Lump of rock cut in half showing crystals formed inside. A geode is a concretion, an irregular or ball-shaped object created when minerals fill a cavity in rock. Typically the mineral deposits form crystals, as seen here. Geodes form in different ways in different rock types. Here the geode formed from a cavity left by a fossilised ball of algae.
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