Silver Overlay

how much is 40 mils of gold plate?

I bought a large gold technibond necklace. its sterling silver with gold overlay. it says 40 mils of gold overlay. is that a lot? I cant tell if thats a good number or bad number for gold plating.
if you can tell me the answer and anything else useful about gold plated jewlery and technibond please do.
thanks!

gold plated jewelry means: there is very very very tiny of gold coated on the other kind of metal ( it can be, silver, lead, coper, etc. ).

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