This is a fossilized insect, one of the Buprestidae (or Splendor Beetles or Jewel Beetles, from the collection of the Hessisches Landesmuseum Darmstadt.
Splendor Beetles are, apparently, one of the longest-living insects in the world. They’re also treasured by collectors for their iridescent shells.
The Messel Pit is a World Heritage Site because it’s just loaded with fossils. Loads and loads of fossils. Some are important fossils. Some are merely beautiful.
This image was uploaded to Wikimedia Commons by someone named Gerbil, but was apparently taken by Torsten Wappler.