Minecraft wiki entry for Development_versions, associated text: Sea Lantern
Prismarine
Wet Sponge
Version exclusive Border block
A red block that has a block model similar to cobblestone wall.
Can only be broken using commands.
Invisible blocks possibly appear on top and bottom of the border block, and they are unbreakable.
Will break if the border block is broken.
Can be obtained using /give.
Obtained item doesn't do anything.
Can be placed using /setblock.
Red particles appear on top of the block and fly upwards.
Version exclusive Chalkboard
Has unused item form textures.
Is listed in autocomplete of /setblock as 2D Wooden Planks icon.
Can only be placed using /setblock. However, it says Block placed, but actually, nothing will be placed.