In a world similar set in renaissance times, there are natural occurring stones with the ability to produce flames. The more precious the stone, the higher and stronger quality of the flame/fire. Each stone, whether it's low quality or not, can be chiseled by skilled craftsmen to produce more efficient fires. These fire stones can also be re-charged in special fire pits till they disintegrate from over use. These stones are mined and excavated for in remote mountains.
Practical use of these stones: If you're in the arctic and need to start a fire, all you need is a few stones and you'll have a blazing fire that lasts longer than firewood. If the stones are of low quality, they can burn for 2-3 hours. If they are superior, they can burn for 2-3 days if left unmolested.
The trouble I'm having is how does one activate such a power? If it's through spoken word (like a spell), what's stopping someone from lighting a wayfarer's stones on fire with the slip of a word? Would it make sense to inscribe runes into the stone that special individuals can activate? What's a more practical way of summoning and putting out the flames? I'm aiming at having these stones being an every-day-use household item used for cooking, warming hearths, etc. — so the simpler the solution, the better.