The Fire Stone is an object that appeared in the book, How to Twist a Dragon's Tale.
Once Humongously Hotshot the Hero, was in love with Valhallarama, her father, Old Wrinkly told him to do an impossible task by stealing the Fire Stone from Lava-Lout Island. As Humongous went there he got imprisoned. He requested his guard Terrific Al to give Valhallarama the ruby stone, which is a signal he is alive. Terrific Al went to her with the stone and as he went back to Lave-Lout Island, he told Humongous that shw was taken by Stoick the Vast.
The Exterminator dragons are starting to invade, because the Lava Stone, which is the prevention of lava flow in that island got stolen. Exterminator eggs hatch when exposed to lava. Hiccup, Fishlegs, Camicazi and Humongous went to the Lava-Lout Island in order to return the Lava Stone. In that island, they met Alvin the Treacherous. He took the Lava Stone. Later, the volcano erupted, since the Lava Stone is not yet returned. Hiccup requested Toothless to get it from Alvin in order to put it in the volcano. Once Toothless did it, the volcano erupted again. This time, the eruption is the hatching of the Fire Stone, which turns out to be a dragon egg. It is a Fire Dragon. It devoured all Exterminators and as it finished, it went in the volcano.
The Fire Stone is shaped like an egg, since it is actually and egg. It likely had an appearance and texture of a stone, since it was mistaken as a stone.
- How the Fire Stone is placed in the volcano in order to hatch parallels the Race to the Edge episode Out of the Frying Pan. In that episode, the Eruptodon egg was placed in the volcano in order to hatch.