The Speaking Stone of Caradoc

The three time travellers make their way east to fabled lands, once part of the continent we know as Atlantis. They must retrieve a Speaking Stone and keep it safe, until it can be delivered to its rightful custodians in the Land of Lyonesse. In the middle of the ocean lies the large island of Atala, the center of the 'Known World'. Atala turns out to be relatively safe and the children explore the capitol, where their knowledge of the Akkadian language improves by the day.
Then it is time to leave through a maze of smaller islands to the 'Land of the Shaking Earth'. They meet witches, who can talk to animals and some crazy royalty, witness a fight between man and beast in prehistoric England and are lost in the future Scottish mountains. But not everybody means well with the children from the future. To just what lengths will giant sorcerers go to get their hands on the Speaking Stone and gain power over the 'Known World'? A spell to protect the time travelers backfires and soon evil dwarfs chase after them in dangerous pursuit.
What exactly is the importance of this unusual stone and why is everybody after it?