Final answer:
Calcium carbonate crystals, like calcite, are found in the teeth of sea urchins, despite also being present in the limestone they grind. The teeth are shaped like needles and plates, contain magnesium, and increase in hardness towards the tip.
Step-by-step explanation:
Calcium carbonate crystals, such as calcite, are commonly found in various organisms including sea urchins and are used as structural materials. One interesting feature of calcite is that it is found in the teeth of sea urchins, even though their teeth are used to grind limestone which also contains calcite. Researchers have discovered that the teeth of sea urchins are shaped like needles and plates and contain magnesium, which increases in concentration towards the tip, contributing to their hardness. This allows the urchins to grind the rock without grinding their teeth.