The merchant grinned “Dear sir, I noticed you had your eye on that charm. Might I interest you in a little bit of its history? Of course I would. I will have you know, that charm has seen its way through many an adventure, and saved the lives of many of the adventurers therein. Legend tells that it was crafted centuries ago by Yurid Norcral, the greated Dwarven runesmith to ever live. Later it was said to have been enchanted by ancient Elven magics which have since been lost to the ages. Quite a rare find.”
A new substance is found in an out of the way area and kept very quiet. It is found to make a perfect counterfeit diamond. Only the most expert of expert appraisers would even have a hope of noticing that it is fake. Only one problem: if it gets wet it starts to slime and if it is saturated, it turns into a glob of gel.