Calcium carbonate (CaCO3) is a very abundant natural biomineral, which inspires researches to fabricate novel optical materials despite calcite crystals are birefringent . Artificial amorphous calcium carbonate (ACC) microlens arrays (MLA) can be synthesized via self-assembly under ambient conditions , by using Ca(OH)2 solution and the organic surfactant polysorbate 20. Upon heat treatment, ACC MLA are crystallized into thermodynamically stable calcite MLA. We analyze structure and properties of the crystallized MLA using synchrotron micro-beam wide angle X-ray scattering (WAXS), polarized light microscopy (POM) and transmission electron microscopy (TEM). By the aid of POM, we show that the MLA crystallize in spherulite-like patterns without changing the shape of individual microlenses. WAXS measurements in the scanning mode allow mapping the local crystal orientations and sizes with respect to the propagating crystallization front. Both TEM and WAXS reveal an average calcite crystal size of 10-20 nm. These nano-crystallites minimize birefringent effects and inhibit undesirable light scattering at grain boundaries . By comparing the elastic strains with the crystallite size distribution of the crystallized MLA, we find smaller nanocrystals in the regions with lower strains . The artificial MLA are based on a strategy for producing complex structures comparable to MLA formed via biomineralization pathways in nature. Crystallization of artificial MLA serves as model system for biomineralization processes. At the same time, we show that utilization of biomineralization strategies in the lab can generate advanced synthetic materials.