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geode

On the outside, a geode looks like an ordinary rock, but when you break a geode open, it's full of crystals.

Geodes are hollow rock formations with minerals, often quartz crystals, massed inside. A geode forms when minerals settle into a hollow in the earth (like an animal burrow) or inside volcanic rock bubbles. First, a hard shell is formed, and then the minerals grow toward the center of the geode, often forming crystals in the process. The Greek root, geodes, means "earth-like."

Definitions of geode
  1. noun
    (mineralogy) a hollow rock or nodule with the cavity usually lined with crystals
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    type of:
    nodule
    (mineralogy) a small rounded lump of mineral substance (usually harder than the surrounding rock or sediment)
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