My guess is, that it is nowhere near 1mm thick and is actually too thin to get a positive reading or it is not real gold. 1mm is very very thick for something that is gold plated. Gold plating is usually measured in microns. A quality watch case that is gold plated may be up to 20 microns thick, that is 1/5 the thickness of a human hair or 1/50th of a mm. Sounds thin right? That actually exceeds (by far) the 2.5 microns required by the FTC for "heavy gold plate". That's my 2 cents.