When you think of where crystals  form, you might imagine some cave deep in the Earth, or maybe  like, actual magma, or something.

But as it turns out, the ocean  is full of crystals, too. And among them is aragonite, which is probably the most important  crystal you have never heard of.

Because aragonite does a lot more than just  look pretty on rock collectors’ shelves. It’s a vital part of marine ecosystems, and it tells us something about life  in the ocean millions of years ago. I promise you this: You're never  going to think about the words "crystal blue water" the same way again. [♪ INTRO] Okay, to start, let’s talk about  what’s going on in the ocean.

Ocean water is notoriously salty,  thanks to a bunch of sodium chloride. But it’s not in the same form in the water  as it is in your pantry or on your fries. The salt crystals dissolve  in water and the charged ions of sodium and chlorine float  around separate from each other.

And there are tons of other ions floating  around in seawater too, that came from other things that go all Wicked Witch  and get melty when they get wet. Two of these ions are calcium, which is  positive, and carbonate, which is negative. Both of those ions contribute  to the ocean’s mild alkalinity.

Which is like the opposite of acidity. When the concentrations of  these ions are high enough, they can come together to form calcium carbonate. And calcium carbonate is what  aragonite is made of, where all the calcium carbonate bits are arranged  in a specific crystal structure.

The crystals have what’s known as an  orthorhombic structure, but that’s not the only shape you can make out of  those calcium carbonate molecules. Another very common form is called calcite. Its crystals have a trigonal structure, which looks a little different  at the molecular level.

There’s also a more rare form of  calcium carbonate called vaterite, which has crystals shaped  like hexagonal-based prisms. Vaterite is more common in mineral  springs than it is in the ocean, and weirdly enough, it’s also found in gallstones. It also isn’t stable over the long term, so it’ll turn into aragonite or  calcite when exposed to water.

Which is why when we’re talking  about calcium carbonate in the ocean, aragonite and calcite are the two main characters. Whether calcium carbonate in the ocean  forms aragonite or calcite crystals depends on the amount of dissolved magnesium that is hanging around in the water. Magnesium ions tend to stop calcite  from forming, so when there’s more magnesium messing with calcite,  aragonite crystals will form more easily.

And while aragonite crystals  can form on their own, they are most often built by  helpful little marine organisms. Molluscs, like mussels, oysters, and sea snails, all make aragonite in order to form their shells. Corals use it when making their reefs.

Even some kinds of seaweed  build a kind of rigid skeleton, using, you guessed it, aragonite. Not only is aragonite the backbone of  marine invertebrate ecosystems, it’s also the building block for some of the most  beautiful things we find in the water, too. The mother of pearl inside oysters?

Aragonite. The pearls themselves? Aragonite too.

Though this one is plastic. There’s even a rare kind of  iridescent gemstone called ammolite, that’s made from fossilised  aragonite in ammonite shells! Aragonite washes up on shore, too.

Lots of beautiful white sandy beaches  that are on your vacation wishlist are almost certainly made from aragonite. Though, how they came to be might make  you rethink your holiday mood board. Because at least 70% of all carbonate  sand in Hawaii and the Caribbean has passed through the gut of a parrotfish.

The parrotfish crunch up hard corals to  get to the tasty organic matter inside, and poop out the indigestible  aragonite in sand-sized chunks. So just, like, try not to think about where that lovely white aragonite sand came  from when it’s between your toes! Anyway, the ocean is a complicated system,  and there is another crucial factor in the ability for these organisms to make  their aragonite bodies - ocean acidity.

See, carbon dioxide in the  atmosphere dissolves into seawater and makes something called carbonic acid. As I mentioned before,  calcium carbonate is alkaline, and it reacts with any acid in the water. This is actually the same thing that  happens in your stomach when you take antacids, because calcium carbonate  is the main ingredient in those, too.

The problem is, this reaction effectively  uses up the carbonate in the ocean, stealing it away from the organisms  that need it for their shells. So when carbonate runs low, the waters  become more acidic, which then starts eating away at the aragonite in seashells  and reefs that have already formed. And acid isn’t the only thing in the  water that can affect aragonite formation.

You might remember earlier when  I was talking about magnesium. And since the presence of magnesium in the water can prevent critters from making calcite, it’s the main thing that keeps the  organisms making the aragonite crystals. And scientists can use the  record of aragonite formation to reconstruct ocean conditions  throughout Earth’s history.

The amount of magnesium in the  ocean changes over long timescales, largely because of changes in  the rate of seafloor spreading. When the movement of tectonic plates  makes lots of volcanic rock, the sea water reacts with the magnesium in that rock,  causing it to leach out into the water. Right now, we have an ‘aragonite sea’, where magnesium is high and the aragonite  is the go-to crystal form for critters.

But about 70 million years ago, during  the Cretaceous period, magnesium levels were lower and calcite was the preferred  crystal, making it a ‘calcite sea’. Other factors can affect the  prevalence of aragonite or calcite in ancient oceans too, including the  sea temperature and the water depth. So geologists can study the crystal  structure of ancient organisms to figure out whether they built their  shells with aragonite or calcite.

Doing that reveals information about  the ocean conditions and chemistry when they lived, which can tell  us about the history of our world. All thanks to this little crystal. This little crystal!

I’m talking to the mic. So that I- I felt like you probably couldn’t hear  me when I was all the way down there. Which goes to show how important the  crystal hitting your mailbox this month is!

SciShow Rocks Box subscribers will be receiving a beautiful specimen of aragonite. Our Rocks Box subscription is a  monthly delivery of a mineral or fossil that we thought was neat enough to send to you. They’re always collected  using best ethical practices for the people in that area, and for the world.

The subscriptions have been  selling out fast, so don’t wait! Visit SciShow. Rocks - and yes, that’s  a real URL - for all the details and to snag a subscription while you still can.

Thanks for watching! [♪ OUTRO]