Avian eggshell is a relatively simple model of biomineralization processes. This biomaterial consists mainly of a mineral part made of columnar calcite crystals and a pervading organic matrix. The organization of eggshell microstructure is determined by genetic, physiological and external factors. Eggshell microstructural characteristics can inform us about biological and physicochemical processes affecting its formation. Quantitative microstructure information can be efficiently determined from two-dimensional diffraction patterns of polycrystalline samples. The present paper analyses the potential of this methodology as applied to microstructure characterization of fully dense polycrystalline materials such as avian eggshell. High correlation was found between parameters determined by X-ray diffraction, such as the number, size and intensity of reflection spots in Debye rings, and crystal size dimensions determined by means of optical microscopy. Crystal sizes can be calculated from these parameters following calibration using samples of the same material whose sizes are already known. Estimated error in crystal size measurements was within 5 %. In comparison with traditional methodologies (e.g., optical microscopy), this technique enabled much faster and more precise determination of microstructural information.