Crystals are light-emitting gems that are found in underground caves. Most crystals have collision (meaning, they are not behind the player, and will stop progress if the player runs into them). Those with player collision will cause damage upon contact.
How to Obtain
You can find different crystals while mining in all biomes. Some crystals are found in every biome, while others are biome-specific. There are no mining skill level requirements or specialized tools necessary to mine a crystal once you locate one.
Crystals also sometimes drop as loot from chests.
The various types of crystals, and the biomes they can be found in, are listed below:
Crystals can be used in crafting, trading, and as decoration. Since they cause damage, you can also use them as a pretty form of protective barrier.
The large red, large purple, and orange crystals have no player collision, but they do have mob collision. This means that even though you can walk right past them, mobs cannot. As a result, crystals can be a creative way to keep mobs at bay, for example, or to contain a pet terrapus in high style.
Since crystals emit a glow, they can also be used as a beautiful, if somewhat expensive, form of lighting.