Xanthine (3,7-dihydropurine-2,6-dione) is a member of the purine family and consists of fused five-and six-membered, heterocyclic rings (Figure ). This chemical 'skeleton' is shared with other purines such as guanine 10 (Figure ) and adenine . Xanthine itself is a metabolic intermediary produced in purine degradation and a precursor in the synthesis of uric acid. It can therefore be found widely in organisms and their waste products. Recently, xanthine has also been found in the form of preferentially oriented layers of biocrystals appearing to backreflect light into receptors in the median ocellus in the Archaeognatha insect group . Given that the majority of biogenic crystals are made of guanine, it is currently unclear why xanthine may sometimes be found instead . Understanding the structure of xanthine crystals will give insight into its role and optical properties and how this might differ from guanine crystal function, providing a greater understanding of why xanthine may form preferentially over guanine.