The most expensive pigment

I just went over most of the painting with a glaze of ultramarine blue and clear alkyd gel. Before the industrial revolution, ground up crystals of lapis lazuli were mixed with oil to create this exact color. The pigmented lapis lazuli was more rare and expensive than gold, and was the used as sparing as possible, and often not at all unless it was expressly requested and contracted by the patron. It wasnt until ultramarine blue was discovered accidentally as a residue in a lime kiln in 1814, with a process for it's production commercially becoming available 14 years later. Now, ultramarine blue is one of the cheapest pigments on the market, priced the same as black and white by most pigment manufacturers.