The empty protein cage of ferritin has been used to synthesize and entrap nanoscale particles of a green cobalt oxide via oxidative hydrolysis of Co(II) by H2O2. The resulting composite material retains the properties of the protein while incorporating the characteristics of the encapsulated mineral. The visible absorption spectrum has a broad band at 350 nm and the IR spectrum shows a band at 587 cm-1 characteristic of the cobalt oxyhydroxide Co(O)OH.