I've had good luck with white's machines, but the best bang for the buck, imho, is the tesoro. They are very fast, very light (that's makes a difference after you swing one for a few hours), and good on batteries. If you have enough $, buy one with a digital readout, otherwise, just make sure it has a good discrimination control - and I think even their cheapest one is pretty decent at that...
Metal detectors send out a radio signal. Just like the radio signal reaching your boombox, that signal penetrates most things with very little problems - BUT if there is any metal there, the signal is blocked. That's why it's very hard to find a good target, like a silver dollar, under a handful of bottle caps. We call it "masking". A really good detector and a lot of experience can help overcome this problem. Some rocks do have metal in them, especially nickel and iron, so, they can be a problem. Others are very reactive to radio signals and are very difficult to detect around (we call them "hot" rocks). Most rocks, though, are transparent (or nearly so) to the radio signals. The biggest problem you usually encounter in a rocky area is damaging your coil by accidentally banging it on a rock. Coils are very sensitive to physical damage - misalign the transmit/receive coil relationship on a vlf detector and the coil is trash.
A metal target responds to a radio signal. Some detectors look for a return signal generated by the movement of electrons in the target (vlf machines). The relative conductivity, shape, and size of the target determine the returned signal, so a vlf machine is easily capable of determining whether your machine "sees" a penny, a nail, a dime, or a silver dollar - and the machine is right probably 90% of the time (remember, for every good target, there is some piece of junk out there that reflects a signal back identical to that reflected by the good target).
Other machines look for changes in the length of time the electromagnetic field takes to collapse back into a coil when the power is shut off because of changes in inductance of the coil (and the size of the field) when a metal target is in the field of a coil. By rapidly switching the power on and off to your coil, and monitoring the time it takes the field to collapse, it is possible to detect a metal object in the field of the coil (this is a pulsed induction, or pi, detector). Pi machines are almost impervious to changes in ground mineralization so tend to "see through" background signal reflection that drives a vlf machine crazy. They are very good, therefore, in salt water or mineralized soils. Unfortunately, they are unable to tell most metals apart (some experience with some machines can USUALLY determine ferrous from non-ferrous, but little else other than the relative amount of metal in the field (target size)).
No matter what detector you decide on, whether a radio shack $79 cheapo or the V3 from Whites at $1700, you'd better plan on using it at least a month BEFORE your big trip. Despite what you might think from watching TV, these things require time to learn what the detector is trying to tell you. Others might disagree with me, but I think at least a few months of using your machine 2 or 3 hours every day is needed before you really learn how to use it.....
). I had to laugh, because, ALL detectors "see through" things (so to speak). So the way your "rocks" question sounded, it seemed along the same lines. Unless you meant to be asking something else?
