Crystals are more common on glacial planets and less common on volcanic planets.
There are four types of crystals: azurite, amethyst, emerald, and ruby. Each comes in 3 sizes: small sliver, medium fragment, and tiny shard. The size of the crystal determines the depth at which it can be found.
Azurite: Appears blue.
Amethyst: Appears purple.
Emerald: Appears green.
Ruby: Appears red.
Small Sliver: The smallest size. Found at depth 0-150. Cannot be crafted. Used to make medium fragments.
Medium Fragment: The medium size. Found at depth 50-200. Can be crafted from small slivers. Used to make large shards.
Large Shard: The largest size. Found at depth 100-250. Can be crafted from medium fragments.