Geneva 3 Quarts/9 Deniers 1785 - Billon Or Silver?

It does look like silver to me. I found an example of a billon coin to compare.

Specific Gravity testing would be adequate to tell the difference between silver and billion BUT in this case the coin weighs only 1 gram. The tolerance of measurements would require an accuracy of approximately 0.0003 grams MINIMUM. I do not know where you would find such accuracy.

This seems to me to be a case for XRF. That test which is also non-destructive can be done but it costs about $30 per test. If the coin is valuable enough - the XRF test is the best candidate.

Numismat The test is now done by PCGS as an add on to their regular grading services. They charge $30 extra but I am not 100% certain they do coins of this type.

Most jewelers do not use XRF the testing apparatus because it is too expensive. You can find them in college physics laboratories and in Nuclear power plant laboratories. But access to these facilities is next to impossible unless you know someone on the inside.

There is one on line coin dealer who used to offer to test coins for about $30 each. I ran into him while doing a google search a year or so ago.

Good luck but if PCGS will certify the coin, I would go that route.