"It's very hard to break the material, because it is a composite between organic and inorganic materials," says Rodriguez-Navarro. "So, if you try to break the purely inorganic calcite it is very simple – it's fragile, so you just hit it, and it collapses. But if you incorporate the organic atoms from the tree sap you make the material tougher. So, the energy you have to spend to break that material is really, really high."