Gold Wind Weed
Invented by the great mage Calypso ages ago, Gold Wind Weed was his solution to find his golden spectacles (which he was always misplacing.) Upon smoking, this weed's smoke moves away in straight tendrils of glowing gold smoke, leading the way toward gold. Each puff will form up to three tendrils of smoke pointing toward the three closest sources, and the smoke will hang in place for five minutes if there is no breeze (only a few seconds if there is any amount of wind.)
The weed itself is typically only purchasable in shops that deal in mining supplies, but a few alchemical shops also trade in it. It's value ranges from 50 gold per pinch to 500, depending upon the perceived naiveté of the buyer. Those that use it know that the draw back is if you are carrying any gold yourself: three gold coins in your pocket will eliminate the value completely.
A single pinch will last for 10 minutes of tracking, and in still air each tendril will extend about 20' visibly (or 50' if you have an exceptional sense of smell.) The weed will remain effective for years, though each year the flavor will deepen to be more like breathing dirt (Dwarves do not actually love this, but Gnomes find it a delicious smoke.)
Strangely it has been discovered by a rare few that chewing this weed and swallowing the juices causes the person's skin to be very sensitive to gold; contact with gold for several hours after this causes extreme pleasure and has a slight healing effect. It has become a "drug of choice" in a few of the wealthiest circles.