Which pigment's synthetic form freed painters from lapis lazuli's price?

Ultramarine. The natural pigment was ground from lapis lazuli, mined essentially in one place - the Sar-i Sang valley in Afghanistan - and shipped across the sea, which is what the name means: beyond the sea. It cost more than gold by weight.

That expense is visible in the paintings. Contracts specified the grade of ultramarine and who paid for it, and the colour was reserved for the Virgin's robe not out of symbolism alone but because it was the most valuable thing in the picture. Vermeer used it heavily, including in underpainting where it cannot be seen, and left his widow in serious debt.

A French industrial society offered a prize in 1824 for a synthetic version and it was claimed within four years. The result was chemically identical and cost roughly a tenth of one per cent as much, and the consequence is that intense blue stops being a statement about money - the Impressionists could put it in shadows and skies without a second thought. The same shift produced synthetic alizarin and mauve, and the collapse of the madder and natural-dye trades with them.