Minecraft wiki entry for Gathering_resources_on_peaceful_difficulty, associated text: Prismarine shards, unfortunately, can only be obtained by killing guardians or elder guardians. However, all of the crafting recipes with prismarine shards: prismarine, dark prismarine, prismarine bricks, and sea lanterns, make up entire ocean monuments, and prismarine and sea lantern also generate in underwater ruins. If you are planning on getting a sea lantern, make sure you mine it with a Silk Touch pickaxe, because otherwise, it will drop only prismarine crystals.