Rocks that set off metal detectors/pinpointers

[Bucket Lister]

Is there a list of rocks & minerals that will set off metal detectors?
I found such a stone today--it was infuriating!!!! 🤬

After several minutes of sifting through substrate with my pinpointer expecting to find another rusty square nail, all that was left was one stone. It doesn't set off the pinpointer like being in contact with a piece of metal, but it does set it off--like being a few inches away.

No, it's not being set off by my keys or pocket change--BT-DT. 😳

 

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[Midden-marauder]

Is that washer-tuning trick confined to Minelab pinpointers?

I haven't run into any brick yet that I can think of, but now that I know about hot & cold rocks (🤦🏼‍♂️), I'll keep it in mind.

In the back of my mind, I figured something like magnetite might do it (wasn't sure about hematite), but I don't run into either of those either.

No, all pinpointers can be super tuned, it's sometimes called the coin trick. I can give you details on how it works if ya like. I guess theoretically any non motion detector could be super tuned but usually it's only used with pinpointers to push them to the limit of depth. I only use the trick on some occasions, depends on what I'm hoping to find and where.
Funny thing happened today, funny coincidence: we were up hiking and we came across an area that I'm reasonably convinced is an abandoned camp ground that, based on context artifacts, probably operated from the 60s to the 80s. As I was poking around trying to nail down dates at the site I came across a hot rock, big one too. Bigger than my hand for sure. Based on the color of the rock I'm assuming it may be rich in iron, red hematite perhaps. It only triggers the pinpointer on one side but it is a bonafide, nature made hot rock! Even my cellphone magnetometer reacts to it

 

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[Almy]

I find quite a few hot rocks in the river cobble. Most are dark and hard and some are brown sandstone. They usually give one strong ghost signal on each side as the detector swings over them, but sometimes just a single signal similar to a copper penny. None have been magnetic. I have not gotten a signal from any of them with my Garrett pinpointer.

 

[Bucket Lister]

I find quite a few hot rocks in the river cobble. Most are dark and hard and some are brown sandstone. They usually give one strong ghost signal on each side as the detector swings over them, but sometimes just a single signal similar to a copper penny. None have been magnetic. I have not gotten a signal from any of them with my Garrett pinpointer.

Huh. My pinpointer is the only way I find them.
Rarely a strong signal; more often, being in contact with one sounds like being several inches away from a valid target.

 

[Midden-marauder]

Huh. My pinpointer is the only way I find them.
Rarely a strong signal; more often, being in contact with one sounds like being several inches away from a valid target.

I think the increased sensitivity to tiny targets found in many pinpointers means they can pick up on some stuff the larger coils are essentially blind to. A tiny piece of iron won't set off my 9 1/2" coil but my pinpointer will sniff it out like a pro if it's within range. The bricks and now one hot rock that I bothered to take home probably contain only the smallest quantity of iron but it's just enough, just enough to set it off. In terms of the red brick I still say that's a potentially useful quality from an archaeology perspective, finding buried brick remnants is an important clue. As for the rest, well, I now collect hot rocks/magnetic rocks when I find ones worthy of comment. Makes for some unusual collecting!

 

[Bucket Lister]

I think the increased sensitivity to tiny targets found in many pinpointers means they can pick up on some stuff the larger coils are essentially blind to. A tiny piece of iron won't set off my 9 1/2" coil but my pinpointer will sniff it out like a pro if it's within range.

Adjusting the sensitivity on your coil doesn't help that?

My 9" coil is picking up everything. Sometimes it can be infuriating, but I have to laugh at myself all the same. I'll get a TID something like 90-94 that eventually turns out to be a ½" long fragment of a wire brad, or ½ cm² piece of foil, 10" deep.

Sometimes finding them in my hand with a pinpointer can be frustrating--almost like panning for gold I imagine, with all the back & forth, and material falling back into the hole.

 

[Midden-marauder]

Adjusting the sensitivity on your coil doesn't help that?

My 9" coil is picking up everything. Sometimes it can be infuriating, but I have to laugh at myself all the same. I'll get a TID something like 90-94 that eventually turns out to be a ½" long fragment of a wire brad, or ½ cm² piece of foil, 10" deep.

Sometimes finding them in my hand with a pinpointer can be frustrating--almost like panning for gold I imagine, with all the back & forth, and material falling back into the hole.

Ya know, in terms of my big coil I'm gonna upgrade before too long, getting into metal detecting was an experiment so I didn't pull for pricy stuff right off. The experiment ended up becoming passion so ya know but.....
I get, on my current coil, often at high sensitivity, a lot of high, 90+ VDI junk signals. Silver doesn't even appear to ring up so high on that machine, deep deep iron, pieces of god knows what at depth or right on the edge of detection range. Hot rocks I suspect, an aluminum can about 14" down perhaps, rusty bits of iron of possibly tiny size, pipes buried 12" down etc. etc. if it rings up over 90 on my machine it's a bit of a mystery and I don't dig those typically, they often report at 8"+ on my machine so they're either just beyond stable detection range or are super tiny just a couple inches down, nothing I want to find. It's a mystery on my big machine, adjusting sensitivity doesn't mean those 90+ signals go away, they just get more shallow or at least that's the assumption. They're trash targets, that's how trash targets show up on my machine. Yes I'm gonna buy a better one but for now that's what appears to be the case. No idea actually

 

[oldkoot]

The hot rock I found is much smaller--I would say less than two cubic inches--and just part of the topsoil.

It's got me thinking though... If someone were interested in actually hunting/collecting these hot rocks, a bank run gravel pit might be worth detecting.

You think one is frustrating you should detect an area that is covered in those little devils, thats what us Nugget hunters deal with all the time

 

[Bucket Lister]

You think one is frustrating you should detect an area that is covered in those little devils, thats what us Nugget hunters deal with all the time

That was just the first, and some time ago. Since then, I'm finding them fairly regularly. More common/dense near the (suspected) privy sites, but can be anywhere. The lightweight ones are graphite; heavier stones... (?)

Where I live & detect, I will NEVER have to worry about finding a nugget. 😉

 

[Bucket Lister]

HOT RED BRICK:

I mostly get grief from bits of magnetite or similar, that's why I'll sweep holes with a pick up magnet, actually saves some headaches. I've been trying to find a comprehensive list of these hot rocks but no luck as of yet. I know red brick will do it but that's potentially useful in my opinion

I have dug up red, tan brown, black brick and have never experienced the brick setting off the detector or the pin pointer.

I haven't run into any brick yet that I can think of, but now that I know about hot & cold rocks (🤦🏼‍♂️), I'll keep it in mind.

Found one! Strong signal--mid 60s; I thought it might be another nickel (found a buff nickel 20 feet away a week ago). My spade hit something (I figured a rock or root as usual). When I pulled the plug out, there was ¾ of a red brick right on the top edge, and it was definitely the target. Set off both Dēus II and MI-6, and when I removed it and double-checked my plug & hole, the signal was gone.

I will say that this brick was "hotter" (louder more frequent beeps from pinpointer) than any of the hot rocks (mostly graphite) I've been finding.

I'm wondering if it's just a matter of whatever clay is/was used in making the bricks containing a high(er) level of iron/hematite.

What about concrete? Sidewalks set off my detector, and I assumed it was iron rebar or mesh reinforcing, but I also recall from school that iron is a component of portland cement, so it's not illogical that concrete, mortar, or anything else containing portland cement might excite a detector.

 

[pepperj]

HOT RED BRICK:






Found one! Strong signal--mid 60s; I thought it might be another nickel (found a buff nickel 20 feet away a week ago). My spade hit something (I figured a rock or root as usual). When I pulled the plug out, there was ¾ of a red brick right on the top edge, and it was definitely the target. Set off both Dēus II and MI-6, and when I removed it and double-checked my plug & hole, the signal was gone.

I will say that this brick was "hotter" (louder more frequent beeps from pinpointer) than any of the hot rocks (mostly graphite) I've been finding.

I'm wondering if it's just a matter of whatever clay is/was used in making the bricks containing a high(er) level of iron/hematite.

What about concrete? Sidewalks set off my detector, and I assumed it was iron rebar or mesh reinforcing, but I also recall from school that iron is a component of portland cement, so it's not illogical that concrete, mortar, or anything else containing portland cement might excite a detector.

I had a concrete re-crushing plant for a customer.
Had a bin for ferrous metals below the Eddy current magnet separation unit.

The nonferrous material was sorted, cleaned by a couple of the crew and brought into the yard separately.

It wasn't just concrete, it was a dumping field for everything metal it seems at a site pour it seemed.

(I know when I was very young my uncle poured a sidewalk, stairs, and landing for their house.
Horseshoes, a bicycle frame, bits of metal went into that landing.🤣)

The guys from the company even showed me banged up coinage, silvers, LCs.

We figured that the old sidewalk rip outs were the ones that held the most coins.

Now I am thinking that the brick might be holding a little treasure.

Brick maker accidentally dropped something in the mold, you found it a 100+ years later.

Break that puppy open and do a recovery.

 

[Bucket Lister]

Now I am thinking that the brick might be holding a little treasure.

Brick maker accidentally dropped something in the mold, you found it a 100+ years later.

Break that puppy open and do a recovery.

I'll see if they still have it--it may have been trashed already.

But IDT there's anything in there. Breaking off a piece should settle things; if all the pieces ring equally, it's the brick itself.

 

[Almy]

I'm going on a trip. There will be little spare room in the car. There will be a possibility for some detecting. I'm thinking of taking just my pinpointer and attaching it to a stick. I don''t want to crawl around on the ground. I plan to try it at home first to determine whether it is feasible. Do you pinpointer-only users have advice for me? Mine is a black Garrett, a fairly old one.

 

[oldkoot]

I'm going on a trip. There will be little spare room in the car. There will be a possibility for some detecting. I'm thinking of taking just my pinpointer and attaching it to a stick. I don''t want to crawl around on the ground. I plan to try it at home first to determine whether it is feasible. Do you pinpointer-only users have advice for me? Mine is a black Garrett, a fairly old one.

all I will say is good luck, should be interesting

 

[bc5391]

I had a concrete re-crushing plant for a customer.
Had a bin for ferrous metals below the Eddy current magnet separation unit.

The nonferrous material was sorted, cleaned by a couple of the crew and brought into the yard separately.

It wasn't just concrete, it was a dumping field for everything metal it seems at a site pour it seemed.

(I know when I was very young my uncle poured a sidewalk, stairs, and landing for their house.
Horseshoes, a bicycle frame, bits of metal went into that landing.🤣)

The guys from the company even showed me banged up coinage, silvers, LCs.

We figured that the old sidewalk rip outs were the ones that held the most coins.

Now I am thinking that the brick might be holding a little treasure.

Brick maker accidentally dropped something in the mold, you found it a 100+ years later.

Break that puppy open and do a recovery.

I've found silver under old asphalt parking lots. The parking lots were no longer used and crumbling, so a bit of digging did no harm.

 

[Midden-marauder]

I'm going on a trip. There will be little spare room in the car. There will be a possibility for some detecting. I'm thinking of taking just my pinpointer and attaching it to a stick. I don''t want to crawl around on the ground. I plan to try it at home first to determine whether it is feasible. Do you pinpointer-only users have advice for me? Mine is a black Garrett, a fairly old one.

You just had to put this bait here didn't you. Of course I've done it and still do it though I'm somewhat less hesitant to use the big coil these days. My thing is that for me the metal detecting game is frequently one of opportunity, I don't always get to choose the when and where, more than half the time I find myself in places where it's worth the time to look but I didn't realize that such a place was on the list of sites to hit. My partner and I are adventurers and we end up in some crazy locations that no one has ever been into with a metal detector or only once ever before I got there kinda thing. Hell, more than half of what we recover isn't even metal so I'm not limiting my tool kit to searching for just one type of thing.
Anyway....
I hope your pinpointer has some reach and I hope you know how to super tune it, I operate a minelab pro find 40 which gets a lot of static because of just how sensitive it actually is, it'll start falsing on some sh*t lemme tell ya BUT I can pick up the utilities with it at 13" so say what you like, minelabs pinpointer has some serious reach. Super tune it and be impressed by just how deep it can go. It's ferrous discrimination ability works alright but ya gotta be close enough to the target to get a steady tone which will depend on target size. I can tell you I dig some bloody tight, clean holes.
Putting it on the end of a walking stick is actually not the worst idea but I'd use a metal pipe clamp to hold it in place as well as a means of super tuning, that'll increase the reach pretty significantly in some cases. Of course you'll either be lugging in a separate pinpointer to close in on targets or taking the one off your stick every time you find something. I personally just bend over to do it, it's easier.
The real strategy boils down to how observant you are, you can only reasonably cover limited areas with a pinpointer so you gotta choose well. You'll not be gridding a large field using this method, period. I look for signs of surface scatter, I look for fire pits, building foundations, encampment sites, obvious indications of human activity focused in a given spot. These are the places the pinpointer comes out and shines. I do find plenty doing it. "Micro sites", that's the best name I've come across for this one and if you're out in the world encountering random locations you didn't even know were there this method actually does work a treat. Less to carry in that's for damn sure

 

[Midden-marauder]

You just had to put this bait here didn't you. Of course I've done it and still do it though I'm somewhat less hesitant to use the big coil these days. My thing is that for me the metal detecting game is frequently one of opportunity, I don't always get to choose the when and where, more than half the time I find myself in places where it's worth the time to look but I didn't realize that such a place was on the list of sites to hit. My partner and I are adventurers and we end up in some crazy locations that no one has ever been into with a metal detector or only once ever before I got there kinda thing. So.....
I hope your pinpointer has some reach and I hope you know how to super tune it, I operate a minelab pro find 40 which gets a lot of static because of just how sensitive it actually is, it'll start falsing on some sh*t lemme tell ya BUT I can pick up the utilities with it at 13" so say what you like, minelabs pinpointer has some serious reach. Super tune it and be impressed by just how deep it can go. It's ferrous discrimination ability works alright but ya gotta be close enough to the target to get a steady tone which will depend on target size. I can tell you I dig some bloody tight, clean holes.
Putting it on the end of a walking stick is actually not the worst idea but I'd use a metal pipe clamp to hold it in place as well as a means of super tuning, that'll increase the reach pretty significantly in some cases. Of course you'll either be lugging in a separate pinpointer to close in on targets or taking the one off your stick every time you find something. I personally just bend over to do it, it's easier.
The real strategy boils down to how observant you are, you can only reasonably cover limited areas with a pinpointer so you gotta choose well. You'll not be gridding a large field using this method, period. I look for signs of surface scatter, I look for fire pits, building foundations, encampment sites, obvious indications of human activity focused in a given spot. These are the places the pinpointer comes out and shines. I do find plenty doing it. "Micro sites", that's the best name I've come across for this one and if you're ou

HOT RED BRICK:






Found one! Strong signal--mid 60s; I thought it might be another nickel (found a buff nickel 20 feet away a week ago). My spade hit something (I figured a rock or root as usual). When I pulled the plug out, there was ¾ of a red brick right on the top edge, and it was definitely the target. Set off both Dēus II and MI-6, and when I removed it and double-checked my plug & hole, the signal was gone.

I will say that this brick was "hotter" (louder more frequent beeps from pinpointer) than any of the hot rocks (mostly graphite) I've been finding.

I'm wondering if it's just a matter of whatever clay is/was used in making the bricks containing a high(er) level of iron/hematite.

What about concrete? Sidewalks set off my detector, and I assumed it was iron rebar or mesh reinforcing, but I also recall from school that iron is a component of portland cement, so it's not illogical that concrete, mortar, or anything else containing portland cement might excite a detector.

Red brick specifically does have higher metallic content than any "non red" varieties. Other types of brick don't set off my detector or haven't as of yet. There has to be enough of said red brick present to get the MD to react at all too.
I don't know about the metal detector but the magnetometer in my cellphone reacts to asphalt, brick and other weird stuff, things with apparent thermoremanence. I like to experiment with it and am often surprised at what it is that has meaningful ability to distort the flux density of the earths field.

 

[Bucket Lister]

....
I don't know about the metal detector but the magnetometer in my cellphone reacts to asphalt, brick and other weird stuff, things with apparent thermoremanence. I like to experiment with it and am often surprised at what it is that has meaningful ability to distort the flux density of the earths field.

Aww, there you go using those highfalutin words...

What's a "cellphone"? 🤣🤣🤣🤣🤣

 

[Chilli]

Red brick specifically does have higher metallic content than any "non red" varieties. Other types of brick don't set off my detector or haven't as of yet. There has to be enough of said red brick present to get the MD to react at all too.
I don't know about the metal detector but the magnetometer in my cellphone reacts to asphalt, brick and other weird stuff, things with apparent thermoremanence. I like to experiment with it and am often surprised at what it is that has meaningful ability to distort the flux density of the earths field.

Yeah the old red bricks from the goldrush era and around that time will often give a signal where I am with detectors.

 

[Midden-marauder]

Aww, there you go using those highfalutin words...

What's a "cellphone"? 🤣🤣🤣🤣🤣

Subsurface geophysical magnetic metal detecting side scanning sonar electrowhatsihoozit thingermaboppy sensor for finding archaeomological features. Ya know, those thingies.
Metal detector stuff there. Sorry, I was both an English major as well as a science major back in school, makes for an interesting combination.

 

[Midden-marauder]

Yeah the old red bricks from the goldrush era and around that time will often give a signal where I am with detectors.

Yeah, I've found that it's red fire brick pretty specifically. I've wondered if it depends on the era the brick comes from but not being a brick expert (even though I'm an expert on things like nails and screws) I can't tell one from another. It's red fire brick, looks old, possibly recycled, it could have been fired five years ago or five decades ago, it's currently beyond me, makes the pinpointer go bonkers, that's all I got