Eggshell this a thin yet relatively strong material due to its composite makeup of ordered calcium carbonate crystals and protein. A layer of crystalline calcite clusters overlays partially aligned calcite columns. The mineral begins as amorphous calcium carbonate nanoparticles, then is transformed to ordered crystals by the presence of C-type lectin-type proteins. The proteins attach themselves to the nanoparticles, initiating crystal transformation, and then detach while crystal growth continues.
"The results show that the chicken eggshell protein ovocleidin-17 can facilitate a transition from amorphous particle to calcite crystal. Intriguingly, strong binding was observed only with smaller nanoparticles (192 formula units); with a larger particle size (300 formula units) the protein consistently desorbed from the calcite phase. Strong binding is regained for very large crystalline surfaces, but in this case mediated by structured surface water...[T]he results lead us to propose that OC-17 acts as a catalyst by binding to amorphous calcium carbonate nanoparticles, transforming them to calcite nuclei, and then desorbing as the calcite begins to grow, thus leaving the OC-17 available to bind to another ACC nanoparticle (see Figure 5)." (Freeman et al. 2010:5136-5137)