In my neck of the woods I come across a fairly wide range terrain from patches of clay (nothing to cry about) to magnetite boulders size of a coffee table and of course areas with very mild ground. Much of my hunting is done with either my Tejon or in all metal mode with the Kruzer or Gold Racer.
Once in a while I would come across a negative hot rock which you can notice especially with all metal mode (not non discrimination mode found in SMF and many other units). There would be a reverse roll to the target loud, blank then loud as if the target bounces. Very obvious characteristics as the signal bounces.

With all the talk about SMF and that useless nickel test made me thing more on fringe targets disappearing in salt such as gold chains etc. Thought what happens if a gold ring was on fringe of detection in salt? Would some of those bouncing signals be a gold ring or just a negative hot rock as I always thought?

So of course in middle of a snow storm I headed to the beach and decided to dig some of those targets. Low and behold, yup they were negative hot rocks.

Brought couple of the buggers home to test and first I put a magnet to them and was able to lift them off the table! They weren't magnetized but had a significant amount of iron or magnetite. Usually hot rocks act bit like coal but these were different. Normal or positive hot rock will spike, in signal but remain in the range of detection. Positive hot rocks have enough mineralization to attract a magnet but not enough to lift it off the surface.

Negative hot rock is unusual where the lowest signal is extremely high on the range of detection and the strongest detection in the middle of the coil as it passes over will cause the target to wrap so the signal falls into the ground phase range where ground balance will erase the target response making it disappear.

Unlike a large silver on edge or stack of quarter trick where target response wraps to iron the negative hot rock is still in mineral range so its phase angle range is just really high.

Understanding this and how an unstable weak target like hot rocks, coal and why their numbers jump leads me to believe that metal targets have enough density and mass to produce a more consistent signal where as less dense targets the signals will vary. This is true to all IB machines no matter the number or chosen frequency.

Back to hot rocks and in particular negative hot rocks, near metal targets. When a negative hot rock is near a coin it will drag the id down but make the id unstable depending on distance from the good target and of course a dd coil will react as it would with a piece of iron but instead of having an iron signal there will be just a lower unstable good target number.

Couple things that do help is if you know there is a negative hot rock simply ground balance over the null. GB will be unusually high but will help with surrounding targets and numbers should stabilize a bit. Also oddly 10 khz seems to help. So in doubt or not sure if a negative hot rock is present as they can be difficult to identify in discrimination modes. 10 khz seems to be less effected than higher or lower frequencies.

Back to gold disappearing in the conductivity of salt... there would be no bounce or wrap in that case as the gold has a stable response and the conductivity of salt will just erase it as the gold goes to the fringe of detection.

Figured maybe couple of ya might find it interesting.