Seashells and chalk are both made of calcium carbonate.

By itself, the mineral is brittle, crumbly, and soft enough to write out a homework assignment.

Seashells, on the other hand, are wonderfully resilient.

They're tough, they resist breaking, and they definitely can't be used to doodle on the sidewalk.

But they're still primarily CaCO3.

So what makes seashells so different?

How can this...and this be made of the same stuff?

We're talking about the mineral chalk, by the way -- sidewalk and classroom chalk are usually made from a different mineral known as gypsum, but we digress.

Both seashells and mineral chalk are the product of living things.

Those chalk deposits were generally formed from the gradual buildup and compression of single-celled ocean organisms with calcified shells.

But bivalve mollusks like oysters and mussels are much more sophisticated shell-builders.

When we use that mineral chalk, it's long dead, and all that's left is the crumbly stuff.

But these mollusks are pulling dissolved carbonate, and calcium from the ocean for some pretty fancy biological architecture.

Seashells contain 95% or more calcium carbonate.

The rest is mostly protein and a bit of sugar.

But a little biochemistry goes a long way.

Calcium carbonate can take on a variety of forms.

Even though its chemical formula never changes, its crystal structure -- which is the 3D arrangement of its atoms in space -- does.

The two forms we care about right now are calcite and aragonite.

Calcite tends to be more stable than aragonite, but many bivalve shells contain a layer of each: calcite on the outside of the shell, aragonite on the inside.

One benefit to layering is that the crystal structures are oriented differently in space.

If the calcite layer fractures, that fracture will probably stop when it hits the aragonite layer, because aragonite doesn't break the same way.

But that alone isn't nearly enough to make seashells so much tougher than chalk.

We told you mollusks were amazing builders.

The aragonite layer in a shell like an oyster's is called nacre, or mother-of-pearl.

Here's how scientists have speculated mollusks actually make the stuff.

Nacre is made up of layers of aragonite crystals separated by very thin layers of protein.

Those layers are anchored by a chain of sugars called chitin.

Mono- and disaccharides, the kind of sugars we eat, are water-soluble and not good building materials alone.

Try to build a castle made of sugar cubes, see how long it lasts.

But chain them together in long strings called polysaccharides, and they can become tough and fibrous.

Chitin is one such polysaccharide.

Proteins attached to the chitin layer give the calcium carbonate crystals a place to start growing.

Those proteins have charged regions that attract the minerals.

Because of how those regions are spaced, the crystals can grow in regular chunks until they hit the next layer and meet each other at the edges.

That's not all the proteins and polysaccharides do.

Shell formation requires a whole army of complex biological molecules, and scientists are still trying to understand how it all works.

Nacre is literally thousands of times tougher than chalk, thanks to the layers of protein and chitin, and the precise structural control.

I don't know about you, but I can't do that.

Oysters, you're so talented!

What all that means is that mollusks don't just wear these minerals as a hat.

They control how the shell is made, and what kind of crystal forms -- either calcite or aragonite.

They can make all sorts of neat shell shapes that don't look anything like a stick of chalk, and they can even remodel their shells throughout their lifetime.

And that's why you can't make sidewalk doodles with a seashell.

Which works out great for the animal, after all.