The skeletons of corals are made of calcium carbonate by biomineralization process, in the form of aragonite or calcite. To understand the characteristics of coral skeletons, especially mineralogy, crystal phases, organization and structure in individual species, X-ray powder diffraction techniques have gained increased interest in recent years as useful non-destructive tools. This review provides an overview on the recent progress in this field and briefly introduces the related experimental approach. The application of X-ray diffraction (XRD) to elucidating the structural and mechanical properties of mineral crystals in corals is reviewed in terms of characterization of CaCO₃ crystal orientation. In addition, we discuss how this technique has increased our understanding of the function of the organic matrix proteins of calcified coral skeletons during mineral formation. Such information is helpful in deducing the mechanical and structural model of corals with respect to biomineralization system of skeletons.