Sea Lantern

"It's dark at the bottom of the ocean. Black, cold, and full of unseen dangers. But a handful of Minecrafters, those brave enough to delve deep below the waves, have reported seeing lights in that darkness. Just before they're attacked by spined monsters with a single, enormous eye. Aargh!"

- Duncan Geere

Sea lanterns are underwater light sources that appear in ocean monuments and underwater ruins.

Obtaining
Sea lantern itself can only be obtained when mined with a tool enchanted with Silk Touch. When broken using anything other than a Silk Touch enchanted tool, it drops 2–3 prismarine crystals. The Fortune enchantment increases the amount of prismarine crystals dropped. This method of obtaining sea lanterns is wasteful though; the 4 prismarine shards and up to 3 of the prismarine crystals used to craft it are non-retrievable.

Natural generation
Sea lanterns generate naturally in ocean monuments as their light source.

Light source
Sea lanterns emit a light level of 15, the brightest possible light level in the game. They produce the same amount of light as glowstone, redstone lamps, jack o'lanterns, and beacons.

Conduit power
Sea lanterns can be used to activate a conduit by building a structure around it. By doing this, the conduit will emit the "Conduit Power" effect in a certain radius, depending on how many blocks are used to power it. Prismarine, dark prismarine and prismarine bricks can also be used for this purpose.

Trivia

 * The animation shows a very faint 'rippling' effect.