The Long Life Stone is an object that appeared in "Smurfquest". It is a crystal, contained in a magic box that can be opened by a special key, which controls the aging process of a Smurf, allowing them to live long lives. However, the crystal's power only lasts for about 1000 years, after which it must be replenished with pure sources of the four primal elements to restore its power. One sign of its weakening power is that the Smurfs start to rapidly age (though with the Smurflings and Baby Smurf it doesn't cause them to grow taller as they age) and the Smurf Village around them starts to deteriorate. Interestingly, the stone's magic also benefits artificially created Smurfs like Smurfette and Sassette, who similarly are affected by the accelerated aging resulting from the the stone's power fading.
Grandpa Smurf had left Papa Smurf 500 years earlier to find where the purest sources of those elements were located in the world, where upon his return the Long Life Stone was reaching the end of its power cycle. With Papa Smurf's help along with Handy, Hefty, Greedy, and Sassette, Grandpa Smurf instantly traveled around the world using one of Homnibus' magic boots to those four places he had found for those elements to collect a sample of each element while the Smurfs left in the village dug around to find the box that contained the Long Life Stone. The latter group had managed to find the crystal before the former group returned with the samples, though at the point of their return, the aging caused by the crystal's waning power was severe to every one but Baby, who only received wrinkles. Using the special key that was hidden in his beard, Grandpa Smurf unlocked the box and used the samples to restore the Long Life Stone's power before he collapsed. Fortunately, the restorative power of the crystal has kept Grandpa Smurf from passing away.
The Long Life Stone was highly sought after by Grandpa Smurf's enemy Nemesis, as it was also sought after by Gargamel when he overheard Papa Smurf calling it their most valued treasure, making the evil wizard think of its value in monetary terms.