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Opal, opalline silica, silica etc. are all essentially the same chemical formula SiO2.nH2O, and guess what... they're EVERYWHERE. No I mean it, you have them in....
Hot springs (picture: https://www.alamy.com/close-up-view-of-sinter-terraces-showing-silica-deposit-detail-waiotapu-geothermal-park-new-zealand-image357599906.html)
as coatings (hyalite) in volcanoes and fumaroles
(image: https://df2sm3urulav.cloudfront.net/tenants/oa/uploads/images/185000-189999/188947/188947_1250230700.jpg)
(image: https://df2sm3urulav.cloudfront.net/tenants/oa/uploads/images/185000-189999/188947/188947_1250230700.jpg)
as biogenic silica in the ocean and on the ocean floor
https://twitter.com/protistologists/status/1250794937903460352?s=20
(Image ©Miguel Méndez Sandín: https://eos.org/research-spotlights/new-recognition-for-major-players-in-the-oceans-silicon-cycle)
https://twitter.com/protistologists/status/1250794937903460352?s=20
(Image ©Miguel Méndez Sandín: https://eos.org/research-spotlights/new-recognition-for-major-players-in-the-oceans-silicon-cycle)
In addition opals have the amazing ability to 
transforminto more crystalline phases like this:
(Image from Lynne et al., 2007, p. 621)
transforminto more crystalline phases like this:(Image from Lynne et al., 2007, p. 621)
Which leads to amazing features like this:
(CL images of sinter from the Bullfrog, Nevada deposit)
(CL images of sinter from the Bullfrog, Nevada deposit)
and this:
(Colour CL image of opalline silica from Crofoot Lewis)
(Colour CL image of opalline silica from Crofoot Lewis)
Oh and did you know that opal coats the inside of geodes before they form beautiful quartz/amethyst etc.
(Image from: https://cdn.shopify.com/s/files/1/2987/3832/products/amethyst-geode.jpg?v=1566990012)
(Image from: https://cdn.shopify.com/s/files/1/2987/3832/products/amethyst-geode.jpg?v=1566990012)
But not only that you can use opal to DATE geological events!
Ludwig et al., 1980
Zielinski, 1980
Neymark et al., 2000
Paces et al., 2004
Nemchin et al., 2006
Amelin and Back, 2006
Ludwig et al., 1980
Zielinski, 1980
Neymark et al., 2000
Paces et al., 2004
Nemchin et al., 2006
Amelin and Back, 2006
That's right, opal can host URANIUM. Which means it can look like this:
(First image Megaw et al., 2018, p. 411, second image my own)
(First image Megaw et al., 2018, p. 411, second image my own)
and this:
(EPMA map of U counts in hyalite opal from Zacatecas, Mexico)
(EPMA map of U counts in hyalite opal from Zacatecas, Mexico)
It's also an incredible fossilizing material:
(Image from: https://www.lindageorgejewellery.com.au/uploads/1/6/7/8/16785236/s981939029293488561_p984_i1_w2560.jpeg)
(Image from: https://www.lindageorgejewellery.com.au/uploads/1/6/7/8/16785236/s981939029293488561_p984_i1_w2560.jpeg)
AND its pretty too look at!
OooooOOOOOooooo
Opals hold a host of information that we can use to determine conditions of past environments, geochemistry of groundwater, biological pathways and cycles, timing of geological events and preserve life through geological time.
I could go on forever and ever, but I am going to plug my @anuearthscience #STUCON2020 presentation on opal which is happening at 1:50PM AEST here: http://anu.zoom.us/j/6066660101 (PW: jaeger)
Do it and learn more from the #RSESrockstars and their amazing and varied research! #STUCON2020
http://rses.anu.edu.au/news-events/events/stucon2020
http://rses.anu.edu.au/news-events/events/stucon2020
