The Carnelian — William L. Stuart

When Aidan and Maggie find a fairy cross while rock-hunting with their grandfather, it's just an oddity. But when they discover there is an elf imprisoned in the stone and set him free, they and their grandparents, Nana and Beebop, are attacked by Dark Elves and forced to flee to the magical world of Celahir. In Celahir, Findecano - the elf the children freed from the fairy cross - leads them on a quest to recover gemstones stolen from the Elven Bow by the Dark Elves. Without the restoration of the gemstones to the Elven Bow, the balance between good and evil in Celahir - and the human world - could tip toward evil.