This fantastic Herkimer Diamond features an enhydro, or more correctly, a fluid filled inclusion.
What's the difference? A scientist will tell you a fluid filled inclusion is fluid, generally water based, sealed within a crystal during its growth. While enhydro does mean fluid filled, this fluid is not necessarily sealed inside. A fracture could have allowed it to enter, and thus escape. In a nutshell, fluid in an enhydro is able to move in and out of the stone, where as a fluid filled inclusion contains fluid that has been trapped inside since it formed; over 300 million years in this case!
This particular bubble of fluid has bits of black hydrocarbon floating inside. Gently move the crystal back and forth to see it. Tough to see with the naked eye, it's clearly seen with a loupe. The 'enhyrdo' shows in the center of the crystal, top photo.
Another cool growth occurrence looks like a gaseous horse shoe, it's seen in several of the photographs.
Also captured in the top photo below is a window facet.
Other natural growth occurrences, including hydrocarbon, are present as well. Rather than deter from this quality Herkimer Diamond, they validate it's natural formation.