Pearls of Lutra, Published in 1996 by Brian Jacques, Philomel Books

"Known as the Tears of All Oceans, six magnificent rose-colored pearls that inspire passion, greed, and deception are mysteriously missing. The only clue is a series of riddles, and Tansy, a young hedgehog maid at Redwall Abbey, is desperate to solve them--for the life of one she holds dear is in great danger.
But Ublaz Mad Eyes--the evil emperor of a tropical isle beyond the horizon--is determined to claim the pearls as his own. And a crew of fearsome monitor lizards and corsairs gathered around him grow restless . . ."