(tungsten. Same density as gold. Wrap a tungsten bar in gold, and you pass weight, density, and surface chemical tests. W is much, much cheaper than gold most of the time.)
Testing a gold bar's electrical conductivity will easily tell the difference between a real and tungsten filled gold bar. So will it's capacitance. Given the simplicity of this test (touch one or two wires to the bar and measure) this doesn't seem a very good technique.
And it's much, much harder to process (harder than most metals, highest melt point). The effort alone would make it at least as valuable as gold in the 19th century.
http://www.popsci.com/diy/article/2008-03/how-make-convincin...
(tungsten. Same density as gold. Wrap a tungsten bar in gold, and you pass weight, density, and surface chemical tests. W is much, much cheaper than gold most of the time.)