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.

World Generation
Prismarine occurs naturally in Ocean monuments along with its variants, where it makes up the majority of the structure. These buildings spawn Guardians and Elder Guardians.

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 appear to slowly change color between brown, blue, gray and purple.