If you’re getting a lot of hits with discrimination and ground balance set and finding nothing of value, it could be due to “hot rocks.” The name has nothing to do with temperature, and everything to do with content.
Hot rocks are rocks that are significantly more or less mineralized than the surrounding ground. A “hot rock” will cause your detector to alert as if it’s found a good target but in actuality, it won’t contain something valuable like copper, silver, or gold. They’re essentially minerals made of iron oxides.
When your detector is ground balanced against the material you’re walking over and you come across these significantly more or less mineralized rocks, they set off an alert that could indicate precious metals. The annoyance of hot rocks is that these potentially valuable target alerts are mostly false alarms.
There are two “branches” of hot rocks.
- Positive hot rocks, which contain a high level of conductive content
- Negative hot rocks (aka cold rocks) which contain high levels of non-conductive content
- Black sand is one form of negative hot rock and its high concentration of magnetite can disrupt ground balancing
Technology Tip: The newer Pulse Induction (PI) metal detectors do a nice job of ignoring hot rocks. The downside is cost. PI detectors cost a lot more than VLF detectors. For VLF detectors, you can adjust your ground balance with some trial and error to try and reject the hot rocks signals.
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There’s not a lot you can do to deal with hot rocks, but here are a few things to consider and try…
Techniques to Deal with Positive Hot Rocks
“Hot” ground causes your detector to chatter, a lot! Sometimes it makes it too hard to tell the difference between the hot rocks and gold that might exist in that spot.
Until you get very acquainted with how your metal detector responds to hot rock areas, it would be better to dig anything you find rather than assume it’s not something valuable. Doing this will help you learn when to dig and when not to.
Positive hot rocks contain a lot of Maghemite, which is also Iron Oxide. That can give them a red or reddish-orange, possibly even yellow color. If you tend to do your metal detecting in one localized region, you could memorize what they look like in that area. See and avoid.
Techniques to Deal with Negative Hot Rocks
Listen for a sort of springy boing sound, typical of negative hot rocks. You can also watch to see how the sound changes as you approach and then move away from a hot rock. Negative hot rocks many times produce a hollow sound, comparatively not as peaked or sharp as a regular metal target.
Negative hot rocks are known to produce alert signals when the search coil is moved in one direction but not the opposite direction.
Practicing with your detector over time will give you the best chance of understanding how it talks to you out in the field. When you’re digging a target, make a mental note of what it sounded like when alerting. After you’ve found a lot of junk and hot rocks, the minor differences in tone will be second nature to you. Then when you hit on something like a gold nugget, you’re quite likely going to know.
Here’s a Summary of Positive and Negative Hot Rock Characteristics
|Positive Hot Rocks||Negative Hot Rocks|
|Reddish-yellow color||Darker color (black sand is one type)|
|Comparatively conductive content||Comparatively nonconductive content, sometimes called “cold rocks”|
|Smaller coils seem to work best (5 or 6 inches), and some say the elliptical Double-D coil shape is best||Smaller coils seem to work best (5 or 6 inches), and some say the elliptical Double-D coil shape is best|
|Smaller in size||Signals are usually heard when the coil passes over in one direction (forward or backward but not usually both)|
|Found near the surface||Response and recovery tone speed tends to be delayed|
|Easy to think these are normal metal targets or possibly gold||Sound is different than a gold target|
|Pinpointers either produce no signal or the signal pulsates|
BONUS VIDEO: Here’s a good video that gives you an idea of how hot rocks can sound different than “normal targets” including gold. He’s using an XP detector, but pay attention to the sound demonstrations he provides at about the 2-minute timestamp.