I made a video of an air test I did with my MXT Pro to show how it can disc zinc pennies and still find copper pennies. Also, the tone is different between the 2! This can be very helpful with target ID but in the real world I dig everything above foil, about +15, so I don't leave possible goodies. But I really like showing off the MXT's ability to spot zincolns! Beware, IH cents can show just below copper cent which so happens to be in the zinc range.
I would like to add a bit of info about tones aiding in target ID. I have noticed when detecting in hotrocks that the tones are high and have a signature "wont beep in the same spot" when swinging over it but the screen is consistantly showing HOTROCK. If your determined to hunt hotrocks, pay attention to high tones that sound off at the same exact spot on the ground because hotrocks will not do that. Hotrocks act like silver worms that move around and you cannot get a consistant beep on each swing. Sometimes it is helpful to close your eyes while swinging over hotrocks and PICTURE in your mind where the target(beep) is, if it beeps in the same spot every time, you need to investigate closer but if its hotrocks it will seem to move around. Note, the detector is already telling you hotrocks, this is just an added comfort food to confirm it really is hotrocks or not.
Anyway, I hope my video helps!