Heavy duty cast bronze. Fits 1/2 inch up to 5/8 inch ground rods. These proper clamps usually cannot be found in building centers. Acorn nuts are not acceptable for use in lightning protection systems. You must use lightning conductor cable to ground lightning protection systems. Only these clamps are constructed properly for use with the proper size conductor cable. Remember, regardless of the length of your ground rod, the top of the ground rod must be buried at least 2 feet below grade.