Another opal rarity, perhaps the rarest, if not fairest, of them all: the opal pineapple.
Opal is an amorphous mineral, meaning it has no crystal struture. Opal pineapples are pseudomorphs. Pseudomorphs occur when one mineral replaces another -- making a cast of the original mineral. In the opal "pineapples," opal has replaced the original mineral - thought to be ikaite.
These oddities have only been found on the opal fields of White Cliffs in New South Wales, and usually range in size from 50 to 200 mm across. They are quite rare and it’s estimated that fewer than 200 have been discovered. Many of the early specimens were cut up into stones because it was more profitable to sell several gemmy opals that one rare opal pineapple.