Under what circumstances can a counterfeit item or a counterfeit alloy produce a positive result?

Under certain circumstances, a counterfeit item may yield a positive—that is, diamagnetic—result on a magnetic scale if the foreign core is hidden too deep inside and the paramagnetic “fake core” is therefore not detected due to the diamagnetic layer. The maximum thickness of a gold layer through which a measurement can be taken is approximately 2.5 mm. Similarly, a coating made of a strongly diamagnetic material that produces a strongly positive value (e.g., bismuth) can, so to speak, offset the paramagnetic core located further inside (which, of course, yields a negative value) and thus skew the reading toward “+.” However, such counterfeits would be difficult to produce, would require a relatively thick layer of, for example, bismuth, and would therefore be detected by a weight or density test.