You're exactly right. Burying a coin in the ground to test is the very same thing as doing an air test, because there's no "halo" around the target caused by years of slow decomposition of the metal reacting with the surrounding soil. Which doesn't mean the test is useless, but it's neither too useful either. Because a coin buried for many years will signal much stronger and at probably twice the depth. And that same "halo" effect is the reason that we can frequently see a classic coin signal suddenly disappear completely once we get the hole dug and re-sweep it. Whenever that happens I get a big rush because I KNOW there's a silver dime or an Indian Head penny in the bottom of that hole.
My recommendation is to take a new machine out and set out a large blanket and put all the jewelry, different coins, and some trash items you can expect to commonly find and separate them out and practice, practice, practice. Sweep the targets, listen to the tones, look at the digital (or Smartfind if you prefer) readings, and pinpoint them. THEN go detecting. Makes the learning curve is a LOT shorter.