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 small, primitive village has yet to discover fire, which is a good thing because there are highly flammable (but not otherwise dangerous) gasses in the air. The heroes, blissful in their ignorance, don't know that and think it a wonderful idea to show the locals open flame.