In the fifteenth century, the alchemist Norton of Bristol said, "Red last in the work of Alkimy." This meant that the final step in changing a metal into gold would be something reddish in color, like an oxygennated metal or mercuric compound. The philosopher's stone, created by guiding a base metal such as lead through color changes would, therefore, be red in color.
In the eighth and ninth centuries, Islamic alchemists thought that all metals were mixtures of mercury and sulfur. They thought that changing lead into gold only involved finding the perfect balance between the two elements. They also used vermillion.
While trying to make the "Red King" as they called it, medieval alchemists used several different compounds. One was the highly expensive and synthetic vermillion, a red pigment made of sulfur and mercury that had been first discovered by the Chinese. Another was cinnabar, a natural mineral form of mercury sulfide.
Some of the last alchemists, Robert Boyle and Sir Isaac Newton, used mercury to create their own rudimentary versions of the stone. Newton suffered from a nervous breakdown shortly after his experiments, and preserved samples of his hair examined in the 20th century point to mercury poisoning as the cause.
Thanks to the April 2002 edition of Natural History.