This Gem Has a Reputation for Mystical Powers

IN MEDIEVAL TIMES, devout Christians thought the red spatterings of iron oxide that dot my chalcedony core owed their origins to drops of Christ’s blood that had seeped into the earth from below the cross. They carved images of the crucifixion and of saints from my hard body (6.5-7 on the Mohs scale). Mystical powers have always been associated with me, back to the days of Babylon. Even today, in a finely powdered form, I’m still used as an aphrodisiac in India.

Who am I?