Amethyst
The name comes from the Greek amethystos, not drunken, and classical writers claimed a cup carved from the stone would keep its owner sober. The etymology may run backwards: the stone is wine-coloured, and the story may have been built to explain the resemblance.
Medieval European sources move amethyst from sobriety to clear-headedness more generally, and it appears in bishops' rings across the period for that reason.
Geologically the colour comes from iron impurities and irradiation in the quartz lattice, and it fades in direct sunlight, which accounts for the pale patches on stones kept too long on a windowsill.