Minecraft wiki entry for Gathering_resources_on_peaceful_difficulty, associated text: Prismarine crystals can either be achieved as a drop from guardians and elder guardians, by breaking a sea lantern, or by finding buried treasure chests. In Peaceful, the only way is to break a sea lantern or find a treasure chest. Sea lanterns can be found in ocean monuments and underwater ruins. You need ocean explorer maps to find ocean monuments, but underwater ruins are much more common; you can even find several ruins during your trip to an ocean monument, and rarely, ruins may also generate on land (similar to shipwrecks). However, the only use of prismarine crystals is to make a sea lantern, which also requires prismarine shards, which can only be obtained by killing guardians, which don't spawn in Peaceful. The only way to get a sea lantern while on Peaceful difficulty is by using a Silk Touch pickaxe.