Here's a breakdown:
* Limestone: Primarily composed of calcium carbonate (CaCO3), with trace amounts of other minerals.
* Chalk: Also composed of calcium carbonate, specifically the fossilized remains of microscopic marine organisms called coccolithophores.
* Shells: Primarily composed of calcium carbonate, either as aragonite or calcite, forming the hard outer layer of various marine creatures like mollusks and crustaceans.
Therefore, calcium is the key element that forms the foundation of these materials.
