BurgerSlave/VenomStone
From Exalted - Unofficial Wiki
Venom Stone
These magical stones are favored by those of high station, who live lives of courtly intrigue and constant peril. Venom stones vary in size, shape, and appearance, and most of them are worked into elaborate jewelry pieces. However, they all share one thing in common--they turn a dark, sickly green if they touch anything that contains poison. The darker the shade, the more deadly the venom. Many a courtier's life has been saved by the discreet application of a venom stone. Thus, crafty assassins who rely on venom contrive to find ways to poison their victims without giving them the chance to use their venom stones.