You get the wishbone from killing Bonemass, the 3rd boss. You equip it like other players have said, and It makes a noise when "treasure" or silver veins are nearby. The more frequent the noise and the higher the pitch, the closer you are to the "treasure" or silver vein. This applies to all zones I believe, but in the mountains I think it just detects silver veins, elsewhere it could be a chest with random items. Also, you may have to dig kind of deep before you hit anything. It's random. I've hit silver after as few as 2 hits with my pickaxe and sometimes it takes closer to a dozen hits.