Actually, they are all Tibetan Skulls by the name of a Kapala. Essentially, in Tibetan Hindu and Buddhist Tantra religions, a human skull is adorned with precious metal and jewel ornamentation and used in rituals.

http://en.wikipedia.org/wiki/Kapala

http://www.google.com/images?q=Kapala&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&um=1&ie=UTF-8&source=og&sa=N&hl=en&tab=wi&biw=1600&bih=696

Hope that helps a bit.