Avian eggshells are composed of several layers made of organic compounds and a mineral phase (calcite), and the general structure is basically the same in all species. A comparison of the structure, crystallography, and chemical composition shows that despite an overall similarity, each species has its own structure, crystallinity, and composition. Eggshells are a perfect example of the crystallographic versus biological concept of the formation and growth mechanisms of calcareous biominerals: the spherulitic—columnar structure is described as “a typical case of competitive crystal growth”, but it is also said that the eggshell matrix components regulate eggshell mineralization. Electron back scattered diffraction (EBSD) analyses show that the crystallinity differs between different species. Nevertheless, the three layers are composed of rounded granules, and neither facets nor angles are visible. In-situ analyses show the heterogeneous distribution of chemical elements throughout the thickness of single eggshell. The presence of organic matrices other than the outer and inner membranes in eggshells is confirmed by thermograms and infrared spectrometry, and the differences in quality and quantity depend on the species. Thus, as in other biocrystals, crystal growth competition is not enough to explain these differences, and there is a strong biological control of the eggshell secretion.

Biological aragonites show various chemical compositions (Sr and Mg) related to the systematic position of the organisms. It is well known that Cnidaria tests have high Mg and Sr aragonites, whereas Mollusca shells have low Mg and Sr aragonites. Sponges have low Mg and high Sr aragonites. The linear correlation between Mg and Sr concentrations is positive. DRIFT spectra show that the wavenumber of a part of the doublet ν ₂ significantly differs in the Mollusca shells and the Sponges and Cnidaria set: 863.7 cm ⁻¹ in Mollusca, 858.8 cm ⁻¹ in Sponges + Cnidaria. The other part of the doublet is similar in the two sets (844.1 cm ⁻¹ ). ν ₁ and the doublet ν ₄ do not seem to be altered. There is a strong linear correlation between the ν ₂ wavenumber and the Mg and Sr concentrations in these biological aragonites. It is known that the calcite–aragonite transition apparently shifts the wavenumber of ν ₂ , but such a shift has not been observed in one mineral.