Power Crystals, created of crystallized energy, can be found below map level 15. These crystals emit low jingle sounds when you are closer than six blocks, which helps you find them. There are 8 colors in total.
All 8 Power Crystals
All colors of power crystals can be found in small like-colored deposits.
Crystals can be broken by anything you want, and you will get them as items.
Natural deposit of purple crystals.
If you put a crystal in a crafting table you get dust from it, which with that dust, you can use in a furnace as fuel (burns 10 blocks), or as an activation crystal .