Depletion gilding is an age old jewelers practice that even the most primitive of metal workers most likely figured out by accident. Basically if you heat and quench an object with some amount of gold in it multiple times the pure gold eventually rises to the surface.

I have an electronic tester that runs a small current through the object. When it works, the most prevalent metal is what shows up. If it is mostly made of alloy then the gold will not register. It requires careful use to be accurate, but it is what I use for most South American material.

To Toftsia: I did not mean to indicate that all of your material was not authentic. It looks like a very interesting group. Much of looks similar to authentic material that I have seen.

Your analysis of gold value is correct from a simple investment point of view. But generally as collectors we are more interested in the origin of an object than the gold content. Everyone has there own approach to such things.

Happy collecting