Ender Portal Frame

The Crystal Block (unconfirmed name) was added to 1.9 Prerelease 3. It has a luminance of 15 and can be placed underwater just like Glowstone (Block).

The blocks cannot be mined, in those terms they are just like bedrock. However if TNT is used it can destroy them to be picked up.

Crystal blocks cannot be set on fire, nor can be torches be placed on them. Regular blocks can be placed. (needs more testing)

The cracked ones (see below in the screenshot) can be repaired, if you rightclick them with an Eye of Ender.

According to the 1.9 Prerelease thread on the Minecraft Forums, this cluster of blocks is known as an Air Portal in the code.