Cerulean Blue Pigment
Discovered in 1805 by Andreas Höpfner, the pigment originally referred to as cerulean blue (or corruleum blue) was first marketed in 1860 as "coeruleum" by George Rowney of the United Kingdom. The primary chemical constituent of the pigment is cobalt(II) stannate.
It is particularly valuable for artistic painting of skies because of the purity of the blue (specifically the lack of greenish hues), its permanence (no other blue pigments retained color as well), and its opaqueness.
Today, cobalt chromate is sometimes marketed under the cerulean blue name but is darker and greener (Rex Art color index PB 36) than the cobalt stannate version (color index PB 35). The chromate makes excellent turquoise colors and is identified by Rex Art and some other manufacturers as "cobalt turquoise".