I just watched a programme on TV about colours, one of which was blue. They showed the rock, Lapis Lazuli, which can only be found in a small number of places in the world including Afghanistan and California.
Lapis Lazuli was used as a paint pigment in paintings by artists like Titian. The rock is ground down to fine powder and then mixed with oil and wax to make oil paint. The resulting colour is called ultramarine and is a superb blue used in things like the blue paint in the robes of the virgin Mary in Titian’s paintings.
So how did Lapis Lazuli become blue? Sulphur rich rock, which was a pale grey brown was heated up 30 or 40 kilometres below the Earth’s surface. The intense heat and pressure apparently compresses the mineral and makes three atoms of Sulphur line up in each molecule. It is this geometry in the molecules that causes it to reflect pure blue light. (Sulphur is usually found in nature in a yellow, red, orange, brown or black form – the Volcanic moon of Jupiter, Io, is coloured by Sulphur).