Yes, hot rocks usually come in at 00-31, coins at 30 or less, mostly in the mid-upper 20's. The general consensus is VERY few good items will be 00-31, which is a GOOD reason, especially if you do not have good eyesight, to use DIGITAL rather than the crosshairs when hunting. It is nearly impossible to see the difference if the crosshair is on 00-31 or 00-29 etc. but the digital number jumps out at you.
Another shortcut- you may have already noticed that the 00-31 reading is almost always a very high pitched STEADY single tone (a monotone high pitched "boiiinnnngggg". It does not waver or warble. Most high-number coins will give what has become called a "flute" sound, as if you are blowing into a flute and fluttering your fingers over the holes causing a series of different sounds, most of them high-pitched in the sweet coin sound range. If a coin is very shallow, it may give also a (blasting loud) monotone signal. After a while you learn to avoid the monotone signal and keep right on going. However, if it is very faint and deep, it deserves closer attention. Some hot rocks also read in the iron range, so if you have some iron discrimination going, you will just get the null sound, loss of threshhold, when you pass over them. PS- when you get really frustrated digging a deep hole in tree roots only to find a crummy modern coin, be sure to toss an old corroded zinc penny back in the hole before filling it, so the next detectorist can experience the same thrill you had.
