Celestial Crystal is a glowstone-like block found in the End on the underside of the floating islands, or in singluar groups on the very tops of the islands. When broken it drops four Celestial Crystal Shards which can be re-combined, similar to glowstone dust.
The shards were once found in the Promised Land before it was removed, and they were light blue rather than purple. The shards were also used to make ambrosia and the old ID was 1963. Five were needed to create a Celestial Lens, required to make a Sacrificial Focus, both of which are now removed.