Across phyla, many organisms self-organize crystals, for functions like vision, pigmentation, and metabolite storage. In zebrafish, a vertebrate known for its crystal-based color patterns, iridophores concentrate purines in membrane-bound organelles, the iridosomes. Inside these vesicles, crystals assemble into large, flat, and thin hexagons following unknown mechanisms that defy typical thermodynamic interactions. Here, we investigate the development of zebrafish iridosomal crystals by using live imaging, cryoFIB-SEM, and novel morphometric analysis pipelines. In doing so, we find that crystal growth predominantly occurs along the b-crystallographic axis, producing their characteristic anisotropic shape. By performing comparative genetic analyses in vivo and reproducing such conditions in silico, we uncover that the zebrafish crystals’ in-plane hydrogen bond molecular structure is the main determinant for the observed crystal anisotropy. Macroscopically, the b-axis anisotropy is controlled by the ratio of guanine-to-hypoxanthine in the iridosome, without affecting the other axes. At the atomic level, the extent of the (100) facet anisotropy depends entirely on the type, number, and strength of molecular H-bonds within the crystal lattice. Mechanistically, our work shows that purine diversity and availability inside the zebrafish iridosome is key to form an anisotropic crystal lattice, leading to the observed functional crystal shapes.