Calcium carbonate consists of three polymorphs that include calcite, aragonite, and vaterite. Calcite is thermodynamically stable under ambient conditions while aragonite is a high-pressure polymorph that is less stable than calcite. Vaterite is the least stable compared to calcite and aragonite and has the ability to transform into one of the other two polymorphs (Guowei et al. 2009).
- Cockleshell: consists of an aragonite form of calcium carbonate which is, interestingly, more biocompatible with better potential to be naturally converted to bone.
Bivalve: The mollusks that have a hinged, two-part shell joined by strong muscles are called bivalves for example clams, cockles, oysters, and scallops
Cephalopods: This group is the most specialized and complex group. The animals in this group are squid, octopuses, cuttlefish, and chambered nautiluses.
Gastropods: The largest group of mollusks for example snails, conchs, abalones, whelks, sea slugs, and garden slugs
An abundant cheap source of calcium carbonate can be obtained from nacreous material.
The nacreous structure is known to be the strongest among all other shell structures (Li et al. 2012; Yin et al. 2016)