In August 2020, I visited The Field Museum of Natural History. It is an amazing place for any one interested in natural history. My main interests were the fossil and mineral collections on display. One specimen that really impressed me was in the Grainger Hall of Gems. It was an opal plesiosaur vertebra fossil from Australia. The Grainger Hall of Gems started off as a Tiffany & Company gem collection exhibited at Chicago's 1893 World's Columbian Exposition. After the exposition was over, it was purchased and made part of the new museum when it opened in 1921.
Over hundreds and thousands of years silica-rich groundwater dissolve fossil remains and leave silica gel. Once the gel hardens it forms opal. Also included in the display are opal fossilized clam and snail fossils.
I have documented other Australian opal fossils at the Natural History Museum in London, England:
https://louisvillefossils.blogspot.com/2020/02/opal-gastropod-fossils.html
and Muséum National D'Histoire Naturelle Minéralogie et Géologie in Paris France
https://louisvillefossils.blogspot.com/2016/08/opal-belemnite-fossils.html
