Prismarine

Prismarine is an upcoming stone-like material that only will appear underwater in Water Dungeons. This block will have several variants, with the rough, gravel-like form as seen in the infobox being one of the variants. The other variants include Prismarine Bricks and Dark Prismarine. The preview image of Prismarine also contains Sea Lanterns, so the two blocks may have something to do with each other.

Properties
The cracks in Prismarine appear to slowly change color between brown, blue, gray and purple.

World Generation
Prismarine occurs naturally in Water Dungeons along with it's 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 composted of prism, referencing how a glass prism creates rainbows out of white light, and marine, since it is an underwater block.