Good catch on the metal detector. However, small stones can also be imbedded in reclaimed wood. All it takes is a little chip in a blade or bit and it's toast, and that's what rocks do. So I also suggest you work over the surface with a wire brush as well. A set of jointer or planer blades for example cost $45-50 and up, a good resaw blade for a band saw runs from $50 to hundreds of dollars. Use slow passes with the metal detector and keep it close to the surface.
The more I do, the less I accomplish.