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Cold And Hot - Rocks Question


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Found an interesting rock today. These are not generally common in my metal detecting areas. It was a nice solid VDI no matter how it was swept or what frequency. Pinpointing was weak and all over the rock but generally strongest in the center. Pinpointing with the TRX at max sensitivity it would sound off weakly unless I centered the tip right on a certain spot. Flipped the rock and tried again with same issue. I estimate it weighs around 15 pounds.

I have half a mind to hump it back to the truck to bring home and slice in two on a wet saw and have a peek inside.

Could someone help me understand why this happens, or point me in the right direction for study, please. Thank you.

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Hot rocks and cold rocks are nothing more than rocks that sound off relative to your current ground balance setting, or are rocks that have conductive properties. When you ground balance you are setting the machine to ignore a certain level of relative iron mineral content in the ground. A rock with dramatically more or less iron mineral content will sound off because it falls well outside your current ground balance setting. You can ground balance on the rock and it will now be silent, except the ground itself will now sound off. The trick with some hot rocks is to average the ground balance setting between where the rocks and the ground both read, helping manage the situation. Most normal hot rocks have some sort of iron mineral content and so give ferrous VDI readings.

Other rocks will have some sort of conductive mineral content, like graphite. These read non-ferrous. There are also rocks with a mix of ferrous minerals, like magnetite, and some non-ferrous mineral, like graphite. They will average out to some extent but whichever mineral predominates will win out.

On the White's system negative numbers are ferrous, positive numbers are non-ferrous. What you probably have is a rock with a higher ferrous mineral content than the basic background that you are ground balanced to - a classic example.

Most people call any rock that beeps a hot rock. However, a hot rock would normally be one that gives a positive "beep-beep" reading, like a nugget or coin. A cold rock give a negative or null "boing-boing" reading as the signal nulls over the target then overshoots.

Easy to demonstrate in person, harder to describe.

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So this is a cold rock since it reads -93 VDI.

If so, I think people using the term "hot rock" for all beeping rocks gave me some confusion. I have found many hot rocks that read the range of +VDI's, so this isolated specimen found while searching clean ground for iron relics threw me for a little loop initially thinking something iron was under it...when there wasn't.

I was thinking this is a cold rock but little is discussed about them. Most discussions revolve around 'hot rocks' from what I saw while searching. Makes more sense now.

Then I started to think, well it is above the ground probe reading at -87 VDI, perhaps it's a hot rock wrapping around the VDI scale... then I just confused myself more.

Pretty sure this is just a quartzite and feldspar common rock with some extra mineralization mixed in strong enough to sound off on my machine.

I think I will recover it for my rock garden as a reminder and learning aid. I'll cut it in half and take a look first. I'll post pictures of the cut rock for anyone else interested.

Thank you, Steve. Your detailed answer is helpful.

Edit: yes, the AM channel goes boing-boing! LOL, that's an accurate description. :laugh:

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For more on rocks that go beep this old but excellent reference. See part two starting on page 29...

  • Advanced Nugget Hunting with the Fisher Gold Bug Metal Detector by Pieter Heydelaar and David Johnson. This out-of-print book is a good basic text on nugget detecting. Although it uses the original Fisher Gold Bug as an example the information applies to most nugget detectors. Part 2 by David Johnson is an excellent primer on hot rocks.
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22 hours ago, Steve Herschbach said:

Most people call any rock that beeps a hot rock. However, a hot rock would normally be one that gives a positive "beep-beep" reading, like a nugget or coin. A cold rock give a negative or null "boing-boing" reading as the signal nulls over the target then overshoots.

 

20 hours ago, Deft Tones said:

I think people using the term "hot rock" for all beeping rocks gave me some confusion. I have found many hot rocks that read the range of +VDI's, so this isolated specimen found while searching clean ground for iron relics threw me for a little loop initially thinking something iron was under it...when there wasn't.

I was thinking this is a cold rock but little is discussed about them. Most discussions revolve around 'hot rocks' from what I saw while searching.

My experience learning the subtleties of the nomenclature very much parallels yours.

Another qualification is "positive" hot rock" and "negative hot rock" -- something Steve details above.  It is unfortunate that misnomers, which come early in a field of study, become common practice/usage, but it happens all the time, particularly in pure science.  It's annoying but, in my experience, nearly impossible to swim against the current.  For example, 'gram' is a unit of mass, not weight.  But almost everyone (including many/most scientists) say "...this specimen weighs XX grams...".

I have a couple large hot rocks (one looks very much like yours!) I found at a GPAA claim between Briceburg and Yosemite in CA, along the Merced River.  The description you give of how the TRX pinpointer reacts is exactly what my Garrett Carrot did -- if you can 'surround' the tip of the pinpointer by as much of the rock as possible it will beep.  Break off a piece and the check it with the pinpointer -- silence.  Both rocks give a 40 ID on my Fisher Gold Bug, which is considered the dividing line between ferrous and non-ferrous, although (see Steve's review of the GB) small nuggets can show up on the iron side of this boundary, so as is always the case, size and shape of the target can have a significant affect on the ID.  My White's TDI/SPP ignores these rocks completely, meaning the response pulse arrives less than 10 microseconds after the pulse was transmitted.  So the detector is acting just as it was designed to do -- ignore the magnetic non-conductors and the very low conducting materials.  (Note:  some hot rocks have resulted in signals on the SPP -- uncommon but not impossible -- so in those cases there is enough conductivity to lead to a longer than 10 us response pulse.)

Another thing I found with both rocks is that I can get a very low resistance reading (few ohms) probing with a multimeter.  This can be interpreted as "highly conductive".  One of them has a very dark, matte finish, consistent with graphite, but the other one (that looks like yours) is more deceptive, even showing some quartz veins.  Both have specific gravities around 2.7 -- quite typical of many rocks but well below most meteorites (darn...).  Although I haven't ruled out the possibility that there is (valuable?) metal buried inside, it seems like a very distant longshot.  If you decide to go to the effort (and expense?) of sawing yours open I'd be curious to see a picture of what you find.  Another option may be to break it apart but I don't have the equipment to do that with mine.  Although I haven't done so yet, I'm sure my non-metal-detecting wife would appreciate a donation to her garden.  Brownie points await.

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I to would be interested in what the interior looks like if you get it cut.  Though the rock pictured is quite dirty I appears to be more of a pegmatite that a quartzite.  I've often had pegmatites sound off on a detector usually with the negative tones.  That was a while back, pre-VDI, however there are some good bits to be found and rock gardens are a great way to train the eye.  *or satisfy the wife as it were :}*

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Was Murphy's Law recovering it and then cutting it but I got it done. The yellowish streak (top right side) coating from the diamond blade - cut freehand. No expense but time, electricity, and 4 gallons hot water (32°F outside). 

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Doing all right Fred :)  Working hard at hardly working and its not working out ;)

Thanks for the cut view Deft :cool:  Now you have bookends Hahh!  A handsome piece of pegmatite.  Coloration is very nice and it does not look like it'd take much polishing to impress the miss'us.

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