Well I can tell you this, on a numbered ID scale the ID Number can vary from one item to the next, meaning not all gold ring up as one absolute number neither does silver.
The reason for varied responses is because of the % of silver and the % of metal it had added to it. Like for example a 10k Gold item could ID different then a 24K Gold item because of the difference in metals.
What I would do is grab some coins and do this:
Get a Nickel and run it over coil see what it reads
Get another (not same one) Nickel and run it over coil see what it reads
Get another (not same one) Nickel and run it over coil see what it reads
See where the different nickels land on your scale, you should see them land in relative same general area, do this with all coins and see what you come up with. (Understand some coins may respond different depending on year of coin)
Gold and Silver Jewelry can give readings all over the place and to try and pin them down to a single number would be futile, but if you get a high reading on something you might want to dig it up just to see what it is, could be a pop can, pulltab or could be a gold/silver ring never know its up to you to dig or not.
