A rather special ammonite from the Majunga Basin, Madagascar. Natural shell displaying striking flashes of blue, green and pink. Cretaceous period circa 100 million years old.  A fossil with added embellishment - the only disappointment with this one it really is too big to have set as a jewel.  Nevertheless an unusual and beautiful thing.
 
Most gem quality opal forms in sedimentary rocks, as it does in Australia, a major source for opals.  Many of these were marine or lake sediments within which shells, bones and wood were sometimes buried and preserved. Most commonly, opal has filled the spaces that remained after these fossils were dissolved; the opal acquired its shape simply because it filled the spaces created by shells or other organic remains.