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Benchtesting Rocks & Minerals With An F75 Metal Detector

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Benchtesting Rocks & Minerals with an F75 Metal Detector


From the earliest time when we were aware of our surroundings, most of us looked for pretty rocks. We wondered what interesting or valuable minerals might possibly comprise them. Now as adult hobbyists, I doubt if any of us hasn’t benchtested an interesting rock from curiosity, and wondered what actually produced the signal. 

Although a sensitive benchtest usually has little in common with how marginally conductive rocks and minerals respond to metal detectors in the field due to ground effects, we can learn and become familiar with how rocks and minerals in our respective areas respond to metal detectors in a benchtest. A sensitive metal detector’s electromagnetic field penetrates rocks, usually generating either a positive or a negative signal in response to whatever material is in the rock. We can sometimes determine whether such signals should be investigated further, or whether worthless iron minerals produced them. 

I’d generally describe my benchtest results as worthwhile and informative, but that notwithstanding, I look forward to doing a benchtest because I think it is an intriguing study on its own merit. That said, how do you conduct a benchtest? I’ll describe my methods and hopefully we’ll see what you think about it.


Benchtest Requirements and Techniques

Benchtesting ideally requires a visually displayed, fully calibrated, manually adjustable ground balance that covers the entire (soil) mineral range from salt to ferrite. As a minimum, the detector should feature a threshold-based true motion all-metal mode, and preferably an additional true non-motion all-metal mode for significantly improved sensitivity to borderline samples. Visual displays in either of the true all-metal modes are essential for target ID, Fe3O4 magnetic susceptibility and GB readouts. 

I prefer a small (concentric) coil to promote detector stability and improve sensitivity to the rock sample, to ensure uniform sample exposure to the coil, and to minimize EMI (electromagnetic interference) especially if benchtesting at home. Elevate the sensitivity control as high as possible while maintaining reasonable detector stability such that you can clearly hear changes to the threshold.

To check for a target ID, move the sample back and forth across the coil at a distance that produces the best signal but does not overload the coil. To determine ground balance and Fe3O4 readouts, advance the sample toward the coil, back and forth to within an inch or two (depending on sample size and signal strength) of the coil’s electrical sweetspot. Ensure your hand does not come within detection range of the coil to avoid creating false signals. If you extend your fingers to hold the sample, this is not an issue when testing larger samples. If necessary use a plastic or wood food holder that can firmly grasp small samples.


Benchtests should be conducted utilizing a minimum of two widely diverse GB control adjustments. Initially I prefer the same GB control adjustment that is typically required to keep my detector ground-balanced to the substrates in my prospecting areas. It’s a personal preference that works for me. That particular GB control point (F75 / GB86) is more likely to improve any rock or mineral sample’s signal strength compared to using a more reduced (more conductive) GB compensation point.

The next step is to use a dramatically reduced GB control adjustment (F75 / GB45) as suggested by Fisher Research Engineering. This setting ensures that (obviously weathered) oxidized samples do not generate a positive signal from any type of non-conductive iron mineral inclusions, particularly maghemite mineralization that may be present within such rocks. It follows that this second benchtest will, if anything, slightly subtract from the sample signal strength, particularly with low grade and otherwise marginally conductive samples, compared to the first step of the benchtest at GB86. 

As a general rule, I do not recommend the F75 / GB45 compensation point for benchtesting (non-oxidized) mafic samples that are dominated by constituents such as common magnetite or other black minerals that normally support highly (non-conductive) elevated GB readouts. Such samples can produce strong negative threshold responses at the reduced GB compensation point. It will be difficult or impossible for the signal from a marginally conductive substance to successfully compete with those negative threshold signals. For non-oxidized samples Fisher Research Engineering suggests using F75 / GB65 rather than the F75 / GB45 compensation point, since obvious iron mineral oxidation should visually be absent from such samples.

With the above discussion in mind, extremely fine-grained, unweathered magnetite that occurs in pyroclastic material (for example volcanic ash) can drop into the GB45 range, but it is extremely rare. Unweathered volcanics do frequently drop into the GB70's due to submicron magnetite, but the recommended F75 / GB65 compensation point will eliminate those positive signals. 


The arsenopyrite sample depicted above is a good example of a commonplace mineral that we encounter in the silverfields of northeastern Ontario. Generally field examples could be described as marginally conductive and many are low-grade. A good many react with only a mild positive signal, and sometimes not at all to a benchtest depending on which GB compensation point is used. 

The high-grade, solidly structured sample above produces a strong positive signal in either zero discrimination or true motion all-metal mode with the ground balance control adjusted to the GB compensation point required for our moderately high mineralized soils. As noted, that’s approximately F75 / GB86, although in the field, of course, it varies somewhat depending on location and coil type / size employed. 

The response is not as strong as a similar size and shape metalliferous sample would produce, but it does generate a surprisingly strong benchtest signal that would be readily detectable in the field. Even with the GB control dramatically reduced to more conductive values (F75 / GB45), to ensure that any positive signals produced by non-conductive iron mineral inclusions should now only produce a negative threshold signal, it is no surprise that this (non-oxidized) specimen continues to generate a strong signal. 


For those readers unfamiliar with detector responses to such minerals, the same general response scenario described above with arsenopyrite applies to other marginally conductive minerals such as galena, pyrrhotite and to a lesser extent even iron pyrites. Ordinary iron pyrites is generally innocuous, but maghemitized pyrite, pyrrhotite, and the copper sulfide ores, particularly bornite and chalcocite, can be a real nuisance in the field due to magnetic susceptibility, magnetic viscosity, and / or electrical conductivity, just depending on what minerals are involved. 

Such variable responses from arsenopyrite and many other mineral and metalliferous examples clearly infer that signal strength and potential target ID depends on a sample’s physical and chemical characteristics, including the quantity of material within a given rock. These factors include structure, size, shape, purity (overall grade), and magnetic susceptible strength of iron mineral inclusions. Moreover, the VLF detector’s sensitivity, the GB compensation points employed, the coil type and size, and the sample profile presented to the coil further influence benchtest target signal strength and / or potential target ID readouts. 

Incidentally, neither of my PI units will respond to the arsenopyrite sample depicted above, even with a TDI Pro equipped with a small round 5” mono coil, the GB control turned off, and a 10 usec pulse delay to deliver its most sensitive detection capability. That result is typical of most, but certainly not all sulfides and arsenides that occur in my areas. Higher grade and solidly structured pyrrhotite, an unwelcome nuisance iron sulfide, and collectible niccolite, a nickel arsenide, are commonplace mineral occurrences here that do respond strongly to PI units, although their respective VLF target ID ranges are quite different. 

As a related but slight diversion, the photo below depicts a handsome example of the widely occurring mineral sphalerite. It forms in both sedimentary beds, and in low temperature ore veins. It is interesting to collectors because it possesses a dodecahedral cleavage which means that it breaks smoothly in twelve directions, and it is usually triboluminescent, meaning that it gives off a flash of light when struck sharply. Like many desirable minerals lurking in prospecting country, unfortunately sphalerite doesn’t react to metal detectors. 


A Final Word

The foregoing is intended to illustrate that sensitive metal detectors can be utilized as a supplementary tool to assist with evaluating rocks and minerals. There is no question that the benchtest has serious limitations, particularly if trying to distinguish positive signals produced by some types of iron mineral inclusions from weak conductive signals. 

That notwithstanding, a positive signal that persists below the F75 / GB45 compensation point cannot be confused with iron mineral negative threshold signals produced at that same compensation point. Therefore a positive signal merits further investigation. Such signals are almost certain to be generated by a marginally conductive mineral or a metalliferous substance. 

On the more interpretive side of a benchtest, we need to point out that weak positive signals from lower-grade samples of minerals such as arsenopyrite, galena, pyrrhotite, chalcopyrite, and doubtless a few others, may disappear well before the GB control is reduced to the F75 / GB45 compensation point. We learn early that benchtests are frequently equivocal and require interpretation based on any further evidence that might support the benchtest result. Look for iron oxidation in addition to structural or other physical evidence as described above that could explain why a sample reacts as it does to a metal detector.


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Hi Simon… thanks for popping in!!! I do think that mineralogy in concert with metal detectors is a fascinating pursuit that without question could keep me occupied for an eternity. My interests incline to the natural sciences, but metal detection was introduced to me by mere happenstance. It is surprising how frequently we see that such trivial chance or unlikely probability determines lifelong interests don’t you think?   

You have several metal detectors well suited to rock and mineral benchtesting. I’m not familiar with your other units, but your T2 and GB Pro should suffice nicely. I fully agree with you that the arsenopyrite is unusually handsome, but then I’m a fool about minerals. 

Benchtesting is technically quite simple, whereas drawing the right conclusions may require access to information about local rocks and minerals. It might be a good idea to acquire a few understandable mineralogy texts. The Petersen Field Guides are excellent references, both the detailed and the simplified versions authored by Frederick H. Pough. I always keep the more compact simplified version handy in an outer knapsack pocket when in the field. A good portion of what we hobbyists learn is self-taught from direct hands-on field experience, hence I can’t overemphasize the importance of understandable field guides.

If you’d prospected here in northeastern Ontario, you’d be familiar with the mineral pyrolusite (MnO2). It’s a general term used to describe a secondary manganese oxide that coats / blackens the surface of manganese-bearing rocks in the tailing piles, shorelines or other surfaces exposed to natural (oxidation) weathering. 

As a point of interest, we have manganese in a reduced ionic state (Mn+1) in our groundwater supplies. It can create laundry-staining issues when it oxidizes (loss of electrons) from Mn+1 to Mn+2. This results when exposed to strong bleaching agents (for example chlorine) because manganese oxidation stains laundry water black. I have an excellent example of pyrolusite but won’t bother with a photo because it is so doggone non-descript and unattractive. I’ve been remiss by not including a silver photo in the article, so below is a small plate silver which is labeled as a “nugget”.


Hi Steve… thanks for stopping around!!! Your comment is most kind, and coming from you it constitutes a very nice compliment indeed. Thankyou for that and for all else that you do on the forum to our benefit.

As to the above article, I wanted to contribute something interesting to the forum. It’s a curious psychological reckoning insofar as you can only read what others contribute for so long, and not experience the need to contribute something in return. 

There were quite a number of minerals whose photos could have been attached to the above article. That was impractical, so depicted below is specular hematite. I don’t know if you encounter this material in the southwest. We have some high production iron mines in several localities, including the renowned surface extraction facility at Marmora, Ontario. Unlike other types of hematite (that I know about) this material, although not exhibiting nearly the full magnetic susceptible strength of magnetite, does seriously react to VLF metal detectors. 


Hi Bob… thanks for dropping by… your comment is too kind. This article directly resulted from our discussions in recent months about mineral identification. Those exchanges prompted me to conscientiously examine some of my samples, many of them lost forever in dusty boxes in the basement. One thing led to another, the keyboard started clattering away one day, and the final result looked appropriate for Steve’s Rock and Mineral sub-forum. 

I don’t recollect where the sample below came from, although it’s undoubtedly from an abandoned site in the Temagami copper district just south of northeastern Ontario’s silver country. I haven’t searched there in some 30+ years.


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Hi Simon… sorry to hear about your neighbor’s workshed. it’s too bad you don’t have their email address or phone # to advise them. They may have expensive equipment that might be exposed if the roof has been breached. Hope everything works out OK.

The rock and mineral book you posted above will probably serve you well, and you will obviously need a local field guide. The Petersen Rock & Mineral guides are easy to read and understand. The advanced guide is a great reference, whereas the simplified version profusely illustrates the basics about rock and mineral formation, crystal forms, simple field ID tests, the individual mineral descriptions presented by category, followed by an index.


You may find that the wealth of minerals described is a bit overwhelming at first, but before you know it, they will be as familiar to you as trusted old friends. You might eventually get yourself a little field portable spectroscope to assist with identification on the more transparent samples. I stand to be corrected, but I think it could also be useful to distinguish between fake and real stones in jewelry. This unit is about the length and twice the width of your little finger. I don’t have one yet but it looks like a handy little gadget once you learn how to use it.

Simon, why don’t you message me your shipping address? I’m thinking about visiting the bookstore. If they have it, I could buy the simplified version and ship it to you ASAP. I don’t mind in the least, it’s only a few dollars, and besides I wouldn’t mind poking around their geology section to see what else is available.


I’ve included two small sample photos of silver above as a result of your comment in your most recent post, so thankyou. We have many fluorite collecting sites in central or eastern Ontario but I have little interest in pursuing it. I think I traded silver for the fluorite sample below. I’m not sure because there have been so many requests over the years that it’s all a muddle now. The time came when my small silver supplies were nearly depleted, and I had to discontinue that practice............................. Jim.  


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Thanks JW for those kindly remarks, and thanks too for all your informative contributions to this forum. I enjoy reading your superb photo-illustrated field reports, the back and forth dialogue between you and Simon, and Simon’s enthusiasm for all things related to prospecting. 

I agree with you that the pursuance of rocks and minerals is primarily about the adventure and discovery. However we can have it both ways John. I like to see my specimens residing on the shelves, they’re like old friends and each one has an associated good memory.

The GB compensation point for the F75 of GB45 essentially accomplishes the same thing as your Falcon except that obviously it doesn’t have the 300 kHz Falcon’s extreme sensitivity. Iron mineralizations will produce a negative threshold response, therefore conductive positive signals produced as the sample is advanced towards the coil should be investigated. The potential issue is if the sample contains both a highly reactive iron mineralization and a conductive substance. Then it remains to be seen which will have the dominant signal. The mortar and pestle is a better solution for such suspect samples, detectors can only tell us so much..........................Jim.


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Hi Simon… I see my PM comments about the Smithsonian Institute Rocks & Mineral guide may have influenced you in selecting that video. I agree with you that most minerals are rather attractive, and chasing after them in the wilds introduces an element of intrigue. However many of the minerals we hobbyists encounter in the field are a considerably lower grade than either museum quality or what we see in mineralogy texts.

I think with persistence that you will eventually find some rhodonite. I’ve been reading about rhodonite occurrences over in New South Wales. Apparently there have been some exceptional gem quality, deep red crystals recovered at Broken Hill, measuring up to five centimeters in length and embedded in galena. 

The photo immediately below is a lithium aluminum silicate called spodumene. Color variations are labeled differently. This colorless, opaque to translucent example is further identified as cymophane. It came to me years ago from a California mineral collector who wanted to trade for some native silver. The second photo is because you seem to like native silver!!! :smile:



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