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Hot rocks
A hot rock is a rock which causes the metal detector to sound off because the rock
contains iron minerals. They come in two basic types.
Negative hot rocks (also called cold rocks) are usually magnetite or contain magnetite,
and give a negative response because their ground balance value is a higher number
than the soil they are found in. They tend to be dark in colour, usually black and usually
heavy. In some cases they will have rust stains. They are usually attracted to a magnet,
and for this reason gold prospectors always carry a magnet – the ultimate
ferrous/nonferrous discriminator. In ALL METAL Mode, negative hot rocks produce a
booing sound rather than the zip sound of a metallic target; recognize the difference
and you will learn to ignore them. As the search coil passes over a hot rock, this booing
sound, or negative response, is quite distinctive. To hear this response, you must be
properly Ground Balanced and in ALL METAL Mode with an audible threshold hum. First,
as the centre of the search coil passes over the negative hot rock, the detector will go
quite; the threshold hum momentarily silences. Then, passing beyond the negative hot
rock, you hear the booing sound. As you pass the search coil back and forth over a
negative hot rock, it will be impossible to pinpoint and will seem as if it moves around.
Positive hot rocks are iron-bearing rocks which have been oxidized by natural
weathering processes so that their GND BAL value is a number lower than the soil they
are found it. They are often small, right on the surface, sound just like a gold nugget, and
are common in many gold prospecting areas. They are usually, but not always, drawn to
a magnet. They are most often reddish in colour but are often black, brown, or yellow.
On relic hunting sites, red clay bricks and rocks which have lined a fireplace or a
campfire will often be hot rocks. The discriminator will usually eliminate them without
difficulty may not quiet them all. In that case, you can revert to the rule of thumb – ‘do
not dig non-repeatable signals”.
Remember to always carry a magnet to help discriminate gold from hot rocks and iron.
• Gold will not be attracted to a magnet.
• Pieces of iron will always be attracted to a magnet.
• Negative hot rocks will almost always be attracted to a magnet.
• Positive hot rocks will usually be attracted to a magnet.
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