A stone believed in the Middle Ages and even later to possess magical or healing powers (apart from the setting or any inscriptions on it). Such beliefs and traditions extend far back into antiquity, but sometimes had some recognizable basis in the nature of the stone, e.g. a transparent hard diamond to make the wearer invincible, a purple amethyst to prevent intoxication, a bloodstone to stop bleeding, an emerald (impervious to light) to aid eyesight, etc. Some minerals and other substances were similarly regarded as magical, e.g. toadstone, adder stone, narwhal ivory ('unicorn horn'), eagle stone, swallow stone, bezoar stone.
Some such substances were administered in powdered form as medicine until the early 18th century.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson