A ring clamp is extremely helpful in holding a ring or other jewelry items while filing it or mounting gems in it. The wedge shoved into the opposite end of the clamp keeps the jaws securely on the jewelry placed in them. The jaws should be leather lined to prevent marring of the jewelry. One end of the clamp is rounded and the other end is squared and both can be used. Most ring clamps are made of a hard wood.
Amazon and Rio Grande have ring clamps for sale on their websites. Otto Frei has metal ring clamps for sale on their website that are equipped to hold a ring on the outside in order to polish its interior. This is not a common style. The wooden ones, like the one pictured, are traditionally and most widely used among jewelers.