My advice is to look for any readings in a positive number. The thinnest gold ring I have found read at only a six, while the biggest heaviest one came in about a 36, which is only one notch from the reading of a zinc penny. So I dig all pennies anyway, in the event that the machine/ground conditions have estimated incorrectly. Besides, the pennies all fit into the piggy bank and they won't if you leave them laying there! Just remember that the machine is making an "estimation" of what it thinks is in the ground. As mentioned by the numbers given above by previous posts, I think the variance of the numbers has to do with the alloy of gold, from 10-18 karats, whether the ring has other metals in it such as platinum, and/or the shape of the ring, the position the ring is laying while in the ground, and whether or not the band is broken or bent. So when you are looking for the gold, dig anything that gives you a consistent number within a notch or two. I have found that if the numbers jump dramatically, I have often dig up foil. Sometimes necklaces/chains will jump around also because the machine is probably trying to read the clumped up links, clasp and/or pendant individually. So, as is often said on this forum....the best discriminator is YOU! I often wonder how many rings and chains I have passed by because I was too lazy, or in too much of a hurry in order to cover alot of ground, to dig the good signals that I had assumed were trash? Doesn't matter, I still find the good stuff and I definitely have ALOTof fun! Happy hunting, and good luck.
Mike