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 ?GIS 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.
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.
Read more: How To Make Fake Gold, ?GIS Journal>>