system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does anybody know if a gold bar is a solid gold bar, and not just a gold-skinned similar metal?

What would be different from a gold bar and a similar-weight-alloy wrapped in a thin layer of pure gold?
gpt
Tungsten have a very similar density to gold, but it is slightly heavier. So it is possible to make a Tungsten bar with a small pocket of resin and a coating of a few millimeter of gold and it would be very hard to distinguish from a gold bar. There is a number of different techniques that have been developed that might be able to detect false gold bars and even gold coins. You may use a scale to check its density, measure the resistance of the gold to detect any thin layers or you might do an ultrasound or x-ray scan of the bar to check for inconsistencies. However none of these techniques are guaranteed to work. Tungsten bars are discovered from time to time by physically destroying the gold bar to check its content. We do not know how much of the worlds gold supply is fake. There are even conspiracy theories that most of the gold in Fort Knox is actually fake and that the US bought tons of tungsten to be delivered to Fort Knox in the 60s.