There is an app which [greatly] assists in determining if government-minted coins are in fact solid AUTHENTIC gold bullion -struck. It listens to the resonance

There is an app which [greatly] assists in determining if government-minted coins are in fact solid AUTHENTIC gold bullion -struck. It listens to the resonance after you tap the coin lightly with a lighter. Something to do with harmonics [I have a fairly good ear for this, myself, but only with well-known currency]. Similar to a digital guitar tuner. But this simple app really left me awe-struck. Many of the laser testing machines can be fooled in certain gold-plated frauds. Gold's "ting" is unmistakable. It's nice to have a virtual co-agreement when dealing in high-stakes quick transactions.