Why was one blue paint once as expensive as gold?

Ultramarine cost as much as gold because it was, quite literally, a crushed semi-precious stone shipped from the other side of the world, and most of that stone ended up thrown away. The pigment came from lapis lazuli mined in Afghanistan, and Italian traders carried it into Europe through Venice in the 14th and 15th centuries. The name still says so: ultramarinus is Latin for "beyond the sea".

Grinding the rock was not enough. Lapis is full of colorless material, so crushing and washing it only gives a dull, greyish-blue powder. To get the real thing, painters ground the stone, kneaded it into a dough of melted wax, resins and oils, wrapped it in cloth and squeezed it in a bath of lye. The blue particles sank to the bottom and the impurities stayed on top. The whole ritual was repeated at least three times, each round giving a poorer blue. The finished pigment cost roughly ten times more than the stone, and the best grades were said to be as expensive as gold.

So painters rationed it like saffron. They saved it for the robes of the Virgin Mary, stretched it with lead white and hid cheaper azurite underneath. Cennino Cennini, a 15th-century painter, called it "glorious, lovely and absolutely perfect". The spell only broke in 1826, when Jean Baptiste Guimet found a way to make it artificially, and a blue once reserved for saints ended up in laundry powder.