Play of Color: Imagine a gem so iridescently beautiful that a phrase, play of color , is coined just to describe it.

Play of Color: Imagine a gem so iridescently beautiful that a phrase, play of color , is coined just to describe it. How does an ordinary sludge of sand and water, boringly described as hydrated silicon dioxide (SiO2. nH2O), express every color in the spectrum of light?

Precious opal is formed when a solution of silica seeps through cracks in a rock very slowly : at a rate of one centimetre thickness every five million years. Under pressure, spheres of silica 150-300 nanometers wide, deposit in crystalline arrays. This regular packing, spaced close enough to the wavelength of light, has the effect of a diffraction grating, and the scattered light can be described by Bragg’s Law. Nearly all the earth’s supply of precious opal comes from Australia, formed in the Cretaceous period, more than a hundred million years ago. 

Eric the Pliosaur: Now imagine a massive thick necked beast that once cruised through the Late Jurassic oceans, with a jaw four times stronger than T. rex and 10 times more powerful than any living creature. 150 million years later, our pliosaur has been “opalized” to an iridescent sheen, his fragments discovered by a lucky miner in Australia’s Coober Pedy and sold for $250,000 USD to a wealthy businessman who subsequently lost his fortune.  Christened Eric the Pliosaur by a mischievous archaeologist who was asked to put the bones together, after Monty Python’s Eric the Half a Bee, the fossil turned out to have a fish inside its belly, fittingly named Wanda. After a public campaign, Eric was eventually purchased for display by the Australian Museum.  What a thrilling journey for Eric. 

► Musical accompaniment: Monty Python – Eric the Half-a-Bee (1972)  

Half a bee, philosophically, must ipso facto half not be .

But half the bee has got to be, vis-à-vis its entity – d’you see?

But can a bee be said to be or not to be an entire bee

when half the bee is not a bee, due to some ancient injury?

► Pliosaurus: http://en.wikipedia.org/wiki/Pliosaurus

► Opalized Fossils: http://www.australianopalcentre.com/fossils.php

#ScienceSunday   #fossilfriday  

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62 Responses to Play of Color: Imagine a gem so iridescently beautiful that a phrase, play of color , is coined just to describe it.

  1. Meg L says:


    Rajini Rao Fascinating! This is the kind of post that makes Google+ sing.

  2. Rajini Rao says:


    Wonderful, happy to share the pleasure Meg L 🙂


  3. Do not forget the most precious: Dragon skin or ammolite.


    I have a tiny piece in my mineral collection in France.


    Note that ammolite looks like opal but is mainly aragonite, CaCO3.


    http://en.m.wikipedia.org/wiki/Ammolite

  4. Rajini Rao says:


    Olivier Malinur ,very interesting. So an ammolite is a fossilized ammonite?


    http://www.amnh.org/science/papers/ammolite.php

  5. Rama Drama says:


    Rainbow shrimps too expensive to eat 🙂 Love the eternal story of how Eric lived with Wanda for millennia(Let’s update Laila Majnu and Romeo-Juliet)!


    Great post as always Rajini Rao and 160 thanks for posting the answer to last week’s mystery photo so I didn’t have to go through every comment to find out: I did go through about 50 of them until it reminded me of my Biology class where I was asked to stand up on the bench for being asleep after reading your conversation with +System Biology 🙂

  6. Rajini Rao says:


    Rama Drama , I’ve been kicked out of biology class too, so you’re in good company 🙂


    Although, my excuse was that I’d already read beyond what the teacher was covering so I was understandably a bit bored. 


  7. Yes, but not the reverse. Not all fossilized ammonites are turning to ammolite. Some can be silicified in opal. It all depends on the fossilization process thus on the diagenetic conditions (basically how the sediments will be transformed in sedimentary rocks). The diagenesis conditions for ammolite is very very special.


    The aragonite of the shell is preserved ! Aragonite is one of these unstable chemical that life loves to use. In normal conditions, it will transform back to calcite. But when buried into bentonite (a variety of clay mineral), it keeps more stable. Now the trick is that bentonite itself is not stable when it gets buried. So you see the story is getting complicated…


    Btw. Opals are also meta stable. It tends to recrystallize naturally.


  8. Fascinating! I had no idea opals were formed in the Cretaceous period!

  9. Rajini Rao says:


    Well beyond my depth, and grateful for the details Olivier Malinur . I thought it interesting that the term opalescence was reserved for the non-precious opal (the milky white kind) which is referred to as potch (a somewhat derogatory sounding term!). 

  10. Rajini Rao says:


    Denis Solaro , it’s funny I meant to write in the post that fossils (ammonite) are rare enough but an opalized fossil is even rarer. Lucky you, to find an ammonite. 


  11. Ammonites are funny fossils. They basically get the accessory mineral of a formation: they can be silicified, opalized, calcified, pyritized and marcassitized…. 😉


    There are some places in Haute Marne and Calvados Denis Solaro , where you can get pyrite ammonite.

  12. Rajini Rao says:


    Denis Solaro , one more reason to live in Provence! Here’s a gorgeous image of opalized wood fossil posted by Rich Pollett : http://goo.gl/fDxZ2M

  13. Rajini Rao says:


    Haha, sorry about the ear worm Mistress Erica . With a name like The Badlands, how can I resist? I’m just discovering what a good website the Royal Tyrell museum has: http://www.tyrrellmuseum.com/index.htm

  14. Jim Carver says:


    Olivier Malinur You are very knowledgeable…I’ve personally forgotten about 3/4 of my mineralogy. I remembered it was hydrated silica but I was thinking it was micro-crystalline or nano-crystalline and I think some forms are.


    I remember Prof. Petersen telling us to keep some water on them from time to time to keep them from desiccating and turning into quartz. 


    Ah, it’s all a buncha chert anyway. 😉

  15. Rajini Rao says:


    Olivier Malinur , I’m discovering that geologists have a penchant for words..”silicified, opalized, calcified, pyritized and marcassitized”. Must look up that last one! 

  16. Jim Carver says:


    Rajini Rao It’s an unstable form of iron pyrite. “Unstable” being a relative term, oc.

  17. Lisa Borel says:


    I stopped on this post for the beautiful opalized fossils, but the observation that geologists have a penchant for words gave me pause… How would one (any scientist or person in any field of discipline) communicate their observations, descriptions and categorizations, if not for words?

  18. Jim Carver says:


    Lisa Borel The good doctor was just kidding.

  19. Marta Rauch says:


    Rajini Rao thanks for this fascinating post! Fun tie-in to Eric the Half-a-Bee 🙂


  20. Brilliant observation! 

  21. Jim Carver says:


    Crystallography is one of the most difficult aspects of geology and chemistry and…


    I reviewed this wiki article and it appears to be kosher.


    http://en.wikipedia.org/wiki/Crystal_systems


    For a through treatment of the subject see Dana’s Manual of Mineralogy


  22. Rajini Rao , we do not pass the biologists in words. But we pass them on wine, beer and various beverages consumption. We need that to keep fit during terrain job.


    Yes, opales don’t support very dry conditions. I said “don’t” because there are various type of opales. I don’t remember the details but during the diagenesis, the sedimentary colloïdal silica will pass from a gel structure to a micro crystalline structure of cristobalite and tridymite. Between the 2, there are various stages. In one of them, there are small spheres of silica, amorphous, with water between. This is the diffraction of light between these spheres which gives the iridescent colours to opales.


    Then there are the black opales, the top of top. They can be faked quite cleverly. They will leave normal opals in a sugary solution. Then apply some acid… The sugar inside the opal will be turned to carbon. As simple….


    And thanks to Pr. Chamley, my professor of sedimentology if I remember all. He made the courses so interesting. That was 27 years ago… Already.

  23. Jim Carver says:


    Mistress Erica No, it’s just shorthand so we don’t have to write “the process of being…” all the time.


  24. Geologists are poets because we can hear the hills breathing, minerals are whispering to us and rocks are telling us legends from the past.


    We have even geopoetry in our vocabulary: about an attractive but still to prove hypothesis.

  25. Jim Carver says:


    Olivier Malinur Haha! Gimme a break. That’s a bit bit overturned I mean over done. 😉


  26. ^^ {giggle} sounds more like a Sunday afternoon reflection of life:)

  27. Clay Newton says:


    Sonja Newton, beautiful!

  28. Rajini Rao says:


    Clearly, geologists rock even though we biologists take them for granite! See how gneiss Olivier Malinur and Jim Carver have been? (They don’t deserve this pun-ishment!). 

  29. Jim Carver says:


    Rajini Rao Sorry I can’t play…I’m just not that sharp today.


    I felt like cubes of pyrite dust, but now it has begun to rust. So there’s little to say, on the terms of decay and hope my opals will stay.


    (Yeah, I know,,,not great, I can already hear groaning in the balcony 😉

  30. Jim Carver says:


    Lucky thing I didn’t try to earn a living as a poet. My iambic pentameter is set to base 2.

  31. Rajini Rao says:


    I’m lava-ing this. Whoever said that a geologist was one rock short of a pile was a lode of schist. 

  32. Jim Carver says:


    The solution here would be to stop pouring more ba-salt on the fissure, otherwise your deposition may be in vein.


  33. I’ve seen many ammonites as opalized and have two recent sightings of them in my Tucson Rock and Gem Show 2013 album.  In person these are just breath-taking.  I’ve seen some animals and other creatures like cephalopods/squids  and they are always amazing.  One of the ammonite photos is here https://plus.google.com/u/0/photos/109294555490324725530/albums/5845455088536848305/5871410248771415442?pid=5871410248771415442&oid=109294555490324725530


    Great post to see, Rajini.  Hope you are sharing this post with #fossilfriday  

  34. Rajini Rao says:


    BJ Bolender , thanks for the link. I knew you would like this 🙂 I’ll edit the post to add the hashtag.

  35. Tommy Leung says:


    I’ve actually “meet” Eric the Pliosaur – there’s a page on the Australian Museum the species which it belonged to – Omoonasaurus demoscyllus http://australianmuseum.net.au/Omoonasaurus-demoscyllus


    I used to do volunteer work at the Australian Museum during my high school and undergraduate years and I’ve handled other fossils bones of vertebrate animals that were opalized, and not all of them were quite as iridescent as those. In fact, they were mostly a dull, dark grey with a slight hint of iridescence.

  36. Rajini Rao says:


    Tommy Leung , I’m vicariously thrilled to know that you’ve met Eric! Although I’m somewhat disappointed that he is a rather small specimen, about the size of a seal. Wiki had led me to believe that pliosaurs were 49 feet long and about 99,000 lbs in weight. I guess he was a young’un. 

  37. Tommy Leung says:


    Rajini Rao yeah, Eric was just a juvenile of a species that didn’t grow all that large. Some genera like Kronosaurus and Liopleurodon were huge (though in the case of the latter, its size was greatly inflated by Walking with Dinosaurs where it was depicted as being 25 m long when in fact it was only about half that length). Funnily enough, I think the spelling of the genus on the Australian Museum might actually be wrong as all the publications I’ve seen about that genus were under Umoonasaurus. I have also met one of Eric’s relatives from the Leptocleididae family – Kaiwhekea – at the Otago Museum http://www.otago.ac.nz/geology/research/paleontology/kaiwhekea-katiki.html while I was living in Dunedin.

  38. Rajini Rao says:


    Confusingly, pliosaurs are different from plesiosuars. I’ve a ways to go before I get the taxonomy right. Besides,  I read that Eric was not a dinosaur at all, but a pinniped reptile?

  39. Tommy Leung says:


    Yep, it is a marine reptile from a group call the Plesiosauria (http://en.wikipedia.org/wiki/Plesiosaur and http://onlinelibrary.wiley.com/doi/10.1111/j.1469-185X.2009.00107.x/abstract), which is an order within a superorder of marine reptiles Sauropterygia call (http://en.wikipedia.org/wiki/Sauropterygia) which lived during the Mesozoic period – at the same time as the non-avian dinosaurs.

  40. Jim Carver says:


    Tommy Leung One specimen? Oh please, do go on. Sorry, my little red devil called skeptic just jumped on my shoulder.

  41. Tommy Leung says:


    Jim Carver hang on, what are you talking about?

  42. Rajini Rao says:


    Jim Carver , there are plenty of fossilized specimen found for the orders we are talking about. For example, in Tommy’s Wiki link above, there is a section under discoveries which lists some of the prominent ones and the range in sizes. 

  43. Tommy Leung says:


    Jim Carver also, read the paper that described Umoonasaurus demoscyllus which is available for free here:


    http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1833998/


    Where it says: “Remains attributable to Umoonasaurus include three partial skeletons from adult individuals (sensu Brown 1981) of around 2.5 m maximum length (based on AM F99374). One additional osteologically immature specimen (SAM P410550) probably represents a juvenile less than 1 m long. The holotype, dubbed ‘Eric’ in the popular press (see Cruickshank et al. 1999), is the most complete opalized vertebrate fossil yet known and is preserved in three dimensions with minimal distortion.”

  44. Jim Carver says:


    I understand that.


     Perhaps more than you give me credit for.

  45. Rajini Rao says:


    Don’t be so touchy, Jim 🙂 You did say that a li’l red devil jumped on your shoulder.  

  46. Jim Carver says:


    Rajini Rao Yeah, things aren’t always usually what they seem. I was just saying I reserve judgment.


    I was born a skeptican….live as a skeptican…and I’ll die a skeptican in Skepticanamerium 😉

  47. Jim Carver says:


    Need a new species of fossil? Try China, they turn ’em out everyday over there.


  48. Fantastic post Rajini Rao . It got me interested in the structure of Opal. It’s amazing how the internal structure gives rise to the different colours, opal expressing every colour of the visible spectrum! Another example of Nature’s wonders where the synthetic opals cannot match the beauty of the natural ones.

  49. Rajini Rao says:


    Thanks, Siromi Samarasinghe ! 

  50. SAN THI says:


    Nice.thank u 4 sharing

  51. Lin Butler says:


    Absolutely lovely!

  52. Rajini Rao says:


    Jim Mac , thanks. Don’t forget the comedy (Monty Python) 🙂


  53. What a … precious, entertaining / educational post Rajini Rao

  54. Jane louise says:


    i could do with these for my fish tank lovely shells

  55. Rajini Rao says:


    Just a few hundred thousand in change, Jane Evans 🙂

  56. Karl Iceton says:


    Wow this is awesome

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