Opals in Polished Ironstone - Scale is difficult to gauge, but, this specimen is about 3 feet tall.
To extract the opal from this boulder would destroy its beauty. Polishing the stone reveals its attractive patterns.
This boulder is from Yoway, Australia.
Natural History Museum in Balboa Park, San Diego Featuring "ALL THAT GLITTERS" - The Splendor & Science of Gems & Minerals.
Exhibition Highlights: http://www.sdnhm.org/archive/exhibits/allthatglitters/highlights.php
***Photos taken without flash through glass/plexiglass