Task: Question Generation
Input (English): In paintings, Mary is traditionally portrayed in blue. This tradition can trace its origin to the Byzantine Empire, from c.500 AD, where blue was "the colour of an empress". A more practical explanation for the use of this colour is that in Medieval and Renaissance Europe, the blue pigment was derived from the rock lapis lazuli, a stone imported from Afghanistan of greater value than gold. Beyond a painter's retainer, patrons were expected to purchase any gold or lapis lazuli to be used in the painting. Hence, it was an expression of devotion and glorification to swathe the Virgin in gowns of blue.
Output (English): What rock was used as the source of blue pigment in paints in Medieval and Renaissance Europe?