The Planet of Jade: Book 4

When the Little Prince and Fox meet the Lithians, a people of stone, trouble is afoot: a menacing mass of brambles is threatening to invade and tear down their city. Jade, the city's ruler, refuses to evacuate. His missing son, Mica, has gone on a quest to bring back the stone-seeds that can protect the city from the attacks of brambles, and he hasn't returned yet. The Little Prince and Fox set off into the dangerous forest in search of Mica…but can they find him before it's too late?