The Seren Stone - The Seren Stone Chronicles, #1

Loren has always loved the fabled stone necklace her mother keeps hidden in her jewellery box. Steeped in lore and legend, it is said that it must be guarded from all who seek it. Unable to resist the lure of the beautiful stone, Loren accidentally invokes its magical properties, catapulting them to another realm. Soon everything is wrong. The Welsh valleys are flooded, the moon is fractured, and dragons-who should only live in storybooks-fill the skies. As Loren and her two siblings are drawn into a future that is barely recognisable, they team up with Orca, a cowardly lechrad, who helps them travel the enchanted land and protect the stone.
They'll need to fight evil and right their wrongs if they ever hope to return home again-and the stone's peculiar magic might be their only chance.