Agate

Agate carries the virtues of truth and immunity to poison. Two carats of powdered agate make a Potion of Detect Lies that works within 20 ft. Combined with a dram of unicorn horn and a carat of jade it becomes a Potion of Immunity to Poison, which protects against every kind of poison but leaves the drinker 10% likely to fumble or trip during combat, running or other hard exertion. Like any gem it may also be one of the seven powdered stones in a potion base.