Which mineral's blue was ground from a single Afghan valley for six thousand years?

Lapis lazuli, from Sar-i Sang in Badakhshan - mines worked since around 4000 BC, and for most of that period the only meaningful source on Earth. Lapis in a Sumerian grave, an Egyptian tomb or a Venetian altarpiece came from the same mountains in north-east Afghanistan.

Grinding it does not give you the colour. Crushed lapis is a dull grey-blue, because the rock contains white calcite and gold flecks of pyrite along with the blue lazurite; extracting the pigment required a laborious process of kneading the powder in a dough of wax and resin under lye, so the heavy blue particles settled out over days. That is why ultramarine cost more than gold, and why the name means beyond the sea.

The mines are still worked and are still a source of trouble. Deposits in Badakhshan have funded armed groups on several sides of Afghanistan's conflicts, and a 2016 investigation traced revenue from illegal lapis extraction to both the Taliban and local militias - a stone that made the Virgin's robe blue for five centuries now functioning as a conflict mineral.