Analyze your site. Figure out the age of your site through research if you can, and then enter the recovered coins and their dates into a spreadsheet. There will be a pattern. Look for it.
One theory is that for every quarter dropped, there should be two and a half dimes, five nickels, and twenty five pennies in the ground as well. I don't really buy that, but the idea is basically sound - people will drop more change with lesser face value than higher face value, and at a certain face value level that depends on the time period in which it happened, they may or may not go back and look for it. Think about it: you'll find pennies all the time on the ground these days, but how often do you see a quarter? Or better yet, a half dollar or a dollar coin? (Granted, nobody uses the latter two coins, but if they did, they would go back and look for them.)
If your site isn't entirely virgin, it's darned likely that one or more people high-graded it in the past few decades, assuming that they didn't completely hammer it. Quarters sound nice; pennies, less so. Nickels are in the trash range and won't often be recovered by cherry pickers. Pennies of all sorts are pretty easy to identify and the folks that don't like digging holes will leave them behind. If all that you have are pennies, nickels, and a few dimes, someone high graded your site at some point. If all you find are pennies, nickels, a few dimes, and quarters from the last ten years, then that's when the site got cherry picked - hence the value of site analysis. You can figure out when the site was picked, who has hit it since, how hard they hit it, and what they were looking for. If you're finding plenty of nickels, that's almost a gimmee that no one has thoroughly hunted the site. If they had, they would have dug up those nickels. (Along with the pull tabs.)
Never fear! That guy did you a favor. When he went through and cleaned up on the quarters, halves, and silvers, he skipped over the gold. Even a very crappy gold ring can buy a lot of silver and/or quarters...probably more than he recovered. That's a lot of holes that you don't have to dig now. Go clean up after him and find all the gold that he missed, and thank him for helping you out like this if you ever get the opportunity.