The Bitterroot Diamonds - Donald F. Averill

With only fifteen months prior to college, David Drum starts a lawn service in Suddenly, Montana, to earn tuition money. His first customer, Mrs. Kincaid, a prototypical grandmother, has an acre of meter-high weeds to remove. David begins and damages his mower on a clay lawn ornament thought to have been discarded a decade ago. Given the clay turtle, he takes it home for repair. His girlfriend, Megan Isaacs, notices a shiny object in the headless turtle and they discover a ring. The local jeweler gives them good news, the ring is valued at $25,000, but it was stolen in 1997. The hunt, assisted by a bogus insurance agent, begins for the rest of the shipment.