Sea Lantern

Sea Lantern is an upcoming block that will only appear underwater. The block will be an underwater light source and gives off a light level of 15. Sea Lanterns first appeared along with Prismarine in a preview. They were added in-game in 14w25a and will be released in 1.8.

Obtaining
Sea Lanterns can be crafted with Prismarine Shards and Prismarine Crystals, or found in Water Dungeons.

As a source of prismarine crystals
When broken using anything other than a Silk Touch-enchanted tool, a sea lantern will drop 2–3 Prismarine Crystals. A Fortune enchantment increases the maximum by 1 per level up to a maximum of 5, with Fortune III increasing the chance of larger drops.