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Millions of years ago, the world was filled with creatures called ammonites.  They were cephalopods with spiral-shaped shells and they ranged from around 2cm to more than 2m across.  Today, they're extinct, but you can find their fossilized shells all over the world.  Ammonite fossils make for some pretty rocks, but there's one place in the world where they've turned into rainbow colored gems.  It's Southern Alberta in Canada and we're not entirely sure why.

When they were alive, ammonite shells were made of a mineral called aragonite.  It's a compound made of carbon, calcium, and oxygen, and in the center of that shell, it combines with organic material to form something called nacre.  You might know it better as mother of pearl.  An ammonite's nacre layer is pretty to begin with, but in certain fossils, it becomes way more vibrant in color.  When that happens, we call it ammolite.  

It's amazing to look at, but it almost shouldn't exist, because aragonite isn't stable over really long time spans, at least at the pressures where ammonite fossils are hiding out.  The atoms normally rearrange themselves into a more stable structure and form the mineral calcite, which is much less fun to look at, but in Southern Alberta, something stopped that from happening. 

The challenge now is to figure out what it was.  To do that, scientists are looking at the Bearpaw Formation, the layer of rock where ammolite is found.  More than 70 million years ago, this area was covered by a vast seaway, but ultimately, volcanic activity filled it with ash.  That killed all the marine life, including several species of ammonites, and further geologic activity buried them properly.  

Scientists don't think this was a normal burial though.  Their evidence suggests the ammonites got buried really fast and were exposed to heavy pressures that could have compressed the aragonite enough to keep it from rearranging itself into calcite.  Additionally, the Bearpaw region had high concentrations of iron and magnesium, and although it isn't quite clear how, that may have helped protect the aragonite further.  Maybe the iron-based minerals formed a sort of shield around the fossils.

In any case, not all of the ammonites found in the Bearpaw Formation went on to become gem-quality ammolites, so whatever the processes were at work, they didn't affect every shell the same way.  Still, the stuff we do have is really gorgeous.  Because of how light acts within it, ammolite can look like a rainbow.  When white light enters thin plates of aragonite crystal, it gets split into its constituent colors through a process called defraction.

The exact color depends on how the crystals are arranged and what impurities exist within the lattice, but you can get some really cool combinations.  Ammolite can even look different colors from different angles, depending on how you see the light pass through it.  Scientists will keep investigating how these fossils formed but one way or another, their story is amazing.  A funny looking sea creature from millions of years ago became a beautiful gem.  

Ammolite is a gift rarer than diamond, which honestly is not actually saying much because diamonds aren't that rare.  If you want to learn more about some minerals and gems that are, though, you can check out our episode about them after this, and always, thanks for watching this episode of SciShow.

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