There are two types of Crystal: Pink and Blue.
Crystal biomes can be found on the world surface, as well as in caves underground.
Pink Crystal only spawns at night, between 20:00 and 02:00 in-game time (at night).
Blue Crystal spawns outside of those hours, in the daytime.
Crystal grass will crystalize into spikes almost anywhere above or underground.
Crystals will not grow within 700 nodes of Spawn.
There is no minimum light for crystal grass to crystalize.
Pink Crystal will spawn along with a Pink Crystal Dirt node; it does not spread like normal grassy dirt.
Sometimes a Salt Crystal will spawn