Prismarine

Prismarine is an upcoming stone-like material that only appears underwater in ocean monuments. This block has several variants, including prismarine bricks and dark prismarine. It makes up a large portion of the monuments.

World Generation
Prismarine naturally spawns in ocean monuments. Deconstruction yields a lot of prismarine, but this is non-renewable.

Crafting
Prismarine can be crafted with 4 prismarine shards in the inventory crafting grid of crafting table. This method requires a decent amount of shards, but is renewable, and the demand can be easily met with a Guardian farm.

Trivia

 * The name "prismarine" was created by Reddit user AjaxGb during a naming contest, where Jeb was looking for a name for the block.
 * Its name is composed of prism, referencing how a glass prism creates rainbows out of white light, and marine, since it is an underwater block.
 * Prismarine appears similar in color and texture to the real-world microcline.
 * The cracks in prismarine slowly change color between brown, blue, gray and purple.