Sharpie Counterfeit Detector Marker with Coil - Magnetic Ink - Black Counterfeit Detector by Sharpie is specially formulated to detect bad bills. The technology reacts to the presence of a key ingredient present in papers that are not used by the U.S. Treasury. The ink marked on a suspect, counterfeit bill will change in color from golden brown to blue-black. An authentic bill will not produce an ink color change. This marker is effective on all U.S. currency. Color: Black