The Guardian's Necklace - Donald Brown - Bog

Josh, the young pirate Guardian, finds himself in danger again when pirates, who were persecuted under the colonist rule, flock under the command of a man calling himself 'The Rebel Prince.'
When they try to seize his Stone of Truth, Josh learns that his stone, which keeps his epilepsy at bay and gives him special powers, is just one of three stones that once belonged to the Guardian's necklace, broken by the Rebel Prince hundreds of years ago.
The necklace was forged by the Guardian as a symbol of peace between the pirates and the colonist invaders. But the prince didn't want peace, he wanted to drive the colonists out of the Western Isles. And now his descendent has the same mad plan.
He wants to collect the three stones, two of which are lost, reassemble the necklace and destroy it in front of his followers as a declaration of war.
Josh sets off on a pirate ship with his friend Megs, and a tiger who lived in the days of Rupert the Rebel, to recover the lost stones, restore the Guardian's necklace and place it in the Temple of Harmony in Amaryllis to restore the peace between pirates and colonists. But not all the passengers can be trusted, and the 'Rebel Prince' is spying on him every step of the way.