Abstract: This invention relates to a method of preparing nanoparticle coated crystals comprising the steps of providing a mixture comprising nanoparticles and a solution of a crystal forming material; and coprecipitating the nanoparticles and the crystal forming material such that crystals are formed, a surface or surfaces of which are at least partially coated with nanoparticles. The invention also relates to nanoparticle coated crystals, a surface or surfaces of which are at least partially coated with nanoparticles wherein the crystal and nanoparticle coating are formed in a single self-assembly step.