Up for auction back in 2010 was this "EXCEPTIONAL GEMSTONE AMMONITE: ...one of the rarest gemstones in the world, formed from the nacreous shell linings of Cretaceous ammonites in only one small area of North America." This particular specimen is even more rare because of the "quantity and quality of these rare colors" and the fact that it "retains areas of excellent three-dimensionality and detail." 20 1/4 inches wide; estimate $40,000-45,000
Sphenodiscus lenticularis, Fox Hills Formation, South Dakota. Late Cretaceous (~ 70 mya). Both sides of this handsome specimen display the extraordinarily intricate suture patterns that are characteristic of this species. Also evident is part of the beautifully iridescent nacreous layer. A laterally flattened ammonite, this animal is thought to have been a relatively fast swimmer.