Secrets of the Gem Trade: The Connoisseur's Guide to Precious Gemstones
“In medieval Europe, superstitions about the religious, talismanic, and medicinal properties of gemstones were accepted without question… The medieval mind, as obsessed as it was with questions of life and death, proved fertile ground for the growth and dissemination of such beliefs. In those times, each gems was valued for its ability to protect its wearer from evils both physical and spiritual… Emerald was considered to protect the wearer against all manner of enchantments… So universal was the belief in the magical and medicinal qualities of gem materials in the Middle Ages, that it is impossible to discuss the value of gemstones without references to them. Was the emerald sought after for its beauty, or for its supposed value as a treatment for diseases of the eyes?”
-Secrets of the Gem Trade: The Connoisseur's Guide to Precious Gemstones by Richard W. Wise