Cobalt blue is a blue pigment obtained by sintering cobalt(II) oxide with aluminum(III) oxide (alumina) at 1200°C. Chemically, the cobalt blue pigment is cobalt(II) alumina or cobalt(II) aluminate, CoAl2O4. Cobalt blue is lighter and less intense than the iron cyanide-based pigment Prussian blue. It is extremely stable and has historically been used as a colorant in ceramics (particularly Chinese porcelain), jewelry and paints. The clear lenses are tinted with Smalt silica-based cobalt pigment.