To discover a fake, you need
to drill or use ultrasound.
Last year I wrote a post about a Manhattan jeweler who was tricked when the gold bar he bought was not 100% gold. (The hollowed-out gold bar scam>>)
L. Burke Files pointed me to an article in his ÆGIS Journal that goes into more detail on how to make and detect fake gold:
To create a top-of-the-line fake gold bar, one that’s capable of being passed off as a real gold bar (the only kind we think worth making), you need to match the color, surface hardness, density, chemical, and nuclear properties of gold perfectly.Read more: How To Make Fake Gold, ÆGIS Journal>>
To do this, you could start off with a tungsten slug about 3 mm smaller in each dimension than the finished gold bar, and then cast a 1.5 mm layer of pure gold all around it. This bar would feel right, it would have a dead ring when knocked (as gold does), it would test right chemically, it would weigh correctly, and it would also pass an x-ray fluorescence scan, the 1.5 mm layer of pure gold being more than enough to stop the x-rays from reaching any tungsten. You’d pretty much have to drill it to find out that it’s fake.