[Toddsterpatriot]

Then again something like an ultrasound or Xray would probably be simplest.

I don't think X-rays would penetrate very far into a gold bar.

[Atlas Sneezed]

I don’t think X-rays would penetrate very far into a gold bar.

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Of course you are right. Still, there must be some kind of acoustic means, like seismologists use. With some kind of gear, that Au/K transition will stick out like a sore thumb.