Mineral Similarities in Seashells and Chalk: Calcium Carbonate
Seashells are made mostly of calcium carbonate, also known as chalk, a mineral soft and crumbly enough to use for sidewalk doodles. Yet seashells are tough and resilient.
In this video, Reactions explains why seashells are so different, and why you can’t use them to draw on your driveway: How Seashells are Made. —American Chemical Society