An old Alchemist (maybe a friend of a PC) is found dead in his laboratory. There is obviously foul play afoot, and the alchemist's working partner/apprentice is nowhere to be found. Further investigation reveals the alchemist might have discovered a way of turning solid elements into gold!
Maybe the apprentice has stolen the formula, murdered his teacher and is hiding somewhere on his own, working on making gold, or claiming the discovery as his own!
Maybe the players will be drawn to find the magic formula themselves?
Maybe they want to avenge their friends death?
Maybe the old alchemist was a bit of a crack-pot, and no-one believes he REALLY found the formula, and just killed himself?
Maybe have the players find the formula, but somehow lose it forever, never to be discovered again…