No single instrument answers every authenticity question. Compare surface, conductivity, magnetic, ultrasonic and dimensional checks before choosing.
Begin with the object
Jewellery usually needs surface-fineness testing. Coins and small bars benefit from conductivity plus weight and dimensions. Larger bars often require ultrasound or a carefully selected multi-method workflow.
Layer independent evidence
A strong result is built from methods that test different physical properties. Match weight and dimensions first, then add a method that reaches the relevant depth and finally investigate any inconsistent point.