A crystal dragon is any type of dragon that integrates crystals into its body in some way.

This can be in the form of crystals on the back, shoulders, and/or tail:

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And/or crystals on the tail tip:

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And/or crystal wings:

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Or simply all over the body:

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Do not tag crystal statues of dragons as crystal_dragon. Do not tag dragons that do not have crystals that look like crystals (if they look like regular plain-colored horns but are canonically "made of crystal", do not tag). For a crystal to look like a crystal, it should have some kind of peculiar way of interacting with light, such as bending or emitting light. Crystals also usually have a pointy shape with sharp edges (this may be sufficient for things like uncolored/unshaded artworks).