The fakes are covered with a fairly thick gold. Some are thick enough to mask the inductance of the core of tungsten. It's particularly a problem with gold bars. 50% tungsten core is enough to make it profitable. . . which costs about $105 to add to real gold casing around it. Sell it as 100% gold Good Delivery Bar (438.9 ounces) for ~$556,000 replacing half of it with Ferro-Tungsten or Tungsten Carbide (~$150). Bank the difference. . . taking whatever risk you think you can get away with about detection using the amount of gold cladding you think is enough.
My DIY pinpointer is sensitive enough to tell per cent of tungsten & gold because it also measures the electric field surrounding the coin against the earth`s field, besides the inductance.