Pathfinder: Shadow of Chernobyl
Canary Stones are a yellow species of carnelian.
They are worn or carried by most people in the Zone as an early warning against extreme Magical Infusion. The gem reflects the amount of raw magic coursing through the body of its wearer, gradually turning darker as the magical energies accumulate. A black Canary Stone generally signifies an imminent death.
Canary Stones take about a day to attune to their wearer.
They are named after the birds that miners used to carry down with them into the coal mines of Chernobyl to warn them if the tunnels were filling with dangerous gasses.