Well, i've had an idea about the Philosopher's stone, as a liquid substance capable to turn vile metals in gold.
Of course, it shouldn't be easy to obtain.
A skill is required to perform alchemy. Let's call it Alchemy, with Plant Lore as a prerequisite.
For the Philosopher's stone you'll need to build an athanor . You can put up to 12 items in the Athanor.
The problem with the Philosopher's stone is that...no one knows how to craft it. You can put everything you want in the athanor, and after some processing you'll obtain 1 liter of a liquid substance called Philosopher's stone. But to see if it is the real stone, you'll need to test the substance on some metal. A nugget will be enough. If you have guessed fewer than 2 ingredients, the nugget will melt down and disappear. You'll get a description of the effect, anyway. Based on the correct ingredients, the nugget should glow more intensly as you approach the real Stone. Also, even ambiental factors could apply. You could have to do the stone in a forest, on a night of full moon. Or you may need to be in a cave in a sunny day. No one knows. You may also need high-quality ingredients to get the correct formula.
If you are lucky enough, you'll finally get the real Philosopher's stone. 0.1 liter of the substance con turn a nugget into gold, and 1 liter can turn a bar of vile metal into a gold bar.
You could argue about a svalutation on the value of gold, right. But it would be a very difficult task to get the correct formula, and whoever find the formula won't tell that to others. Since we are talking of very rare ingredients, he'll need to find or buy them, risking to get suspected if he get large quantities. If he get too much gold fastly he'll be suspected too, so he should care about his treasure and try to keep it a secret.