Guanine crystals are frequently encountered in nature in the beta-polymorph to create structural colors, to enhance the vision of creatures, and for camouflage. Unfortunately, it is challenging to control the crystallization of guanine in aqueous conditions in the laboratory because of its low solubility in water. Here, we crystallize guanine in an aqueous environment under confinement. We employ water-oil-water double emulsions stabilized with a metal-ligand functionalized surfactant as selectively permeable containers to crystallize guanine by dynamically adjusting the pH and guanine concentration. If formed under high osmotic pressures that result in high guanine concentrations within emulsion cores, guanine crystallizes into the anhydrous beta-polymorph with a spherical morphology. In contrast, if crystals form within emulsion cores containing low guanine concentrations, they attain the monohydrate form possessing a needle-like morphology. These findings demonstrate for the first time that the structure and morphology of guanine crystals formed in the laboratory under confinement in an aqueous environment can be tuned by the local guanine concentration and to some extent by the solution pH.