Jump to content

Mystery Rock / Mineral Found While Detecting


Recommended Posts

I was in a schoolyard searching for old coins when I got a 20-21 hit (zinc penny zone) but also a weak signal (5 bars on the Equinox 'depth' meter).  Since Indian Head pennies and early Lincolns can TID there, weak signals are always worth digging.  About 5 inches down out popped a dirty piece (metal?) which felt light, so not lead.  At first I thought it might be melted aluminum but it sure doesn't have the right shape.  It appears to my eye to be naturally formed.

mystery_mineral.thumb.JPG.587e44adf17007a377290330e1121ac2.JPG

After arriving home and washing it off it still looked natural so I did some more investigation.  Here is what I found:

0) non-magnetic (Nd super magnet used for this test).

1) specific gravity measured (by water offset method) at 2.65.  That may be a tad bit low since I didn't correct for the buoyancy of the wire bale that I used to hold it in suspension.  So let's say 2.70 +- 0.10.

2) left a grayish (possibly olive green) streak.  It was not as dark as a graphite pencil streak but still quite distinguishible.

3) easily scratched calcite but not fluorite so Mohs hardness at least 3 and less than 4.

4) air tested dTID from 61 to 70 on a Fisher F75 (the zinc penny sweetspot is 65 so this is consistent with what the Equinox read in the ground).

5) gave a strong signal (up to 6" distance) on my White's TDI/SPP (TDI/SL sister) pulse induction detector with Miner John 5"x9" folded mono, gain = 5 (i.e. midpoint), ground balance off.  I tested some other minerals of similar size:  chalcopyrite, galena, pyrite, graphite, and lodestone (magnetite).  Only the lodestone responded on the White's TDI, and only within about 1.5 inches of the coil.  A copper penny gave similar response to about the same 6" distance from the coil.  (Battery potential measured at 11.75 V.)

I thought it might be the mineral chalcophyllite, but the hardness is wrong for that.  (I have no idea if that mineral would give a signal on a PI detector.)  AFAIK my area isn't known for copper minerals (or any minerals worth mining other than limestone and coal).  Glaciers did bring down various rocks and minerals from (now) Canada and the Great Lakes (including fine gold, native copper, and small diamonds) so I guess that is a possible source.  I haven't ruled out aluminum.  An aluminum (alloy) beavertail pulltab gives a similar streak.  But the specimen's density is low for an alloy and its hardness is low for pure aluminum (Mohs hardness of 2.75) even though pure aluminum has the right specific gravity.

Any ideas on futher tests (besides X-ray fluorescence spectroscopy since I don't have access to one of those guns)?  Try to saw it and look at the inside??

  • Like 3
Link to comment
Share on other sites


Nice Barber, GB. 🙂

I find a lot of odd metal like that, usually it's heavy so I know it's lead, that is what it looks like. 🤔 Found some odd stuff myself today, including a large green rock. I had a large piece of lead come up as a 22!

  • Like 1
Link to comment
Share on other sites

Looks like white metal which is usually mostly zinc. Think car hood ornaments and fake hood grills on the cars from the 70's. The way your piece looks corrosion wise, I would say zinc based and sometimes also referred to as pot metal.

  • Like 1
Link to comment
Share on other sites

I was hoping for more responses, and maybe they will trickle in.  People who have spent thousands of hours searching for gold should be able to provide insight.  Just saying "never heard of a mineral with these properties" would be helpful.

23 hours ago, schoolofhardNox said:

Looks like white metal which is usually mostly zinc.

Good observation.  The problem is the density.  Most common metals (iron, copper, zinc and their alloys) have specific gravities in the 7-9 range and that's true for zinc as well, at 7.13.  I've found a fair amount of zinc items.  Canning jar lids seem to be fairly common in sites frequented prior to the early 1960's.  And I agree with you that many automobile (and truck) parts from earlier days were zinc or zinc alloy.  I've found my share of those.  It seems quite often the ground causes the zinc to form a white granular scale which comes off easily, without even trying.  (And don't forget the hated Stinkin' Zincolns!)  I don't know the SG of pot metal but I'm pretty sure it's also much higher than 2.7.

As I review my 6 enumerated datapoints (and having slept on this 😁), Occams Razor says I have a piece of aluminum.  If it's an alloy that could explain the discrepancy in hardness.  I had mentioned a problem with specific gravity but there are many aluminum alloys.  I was referring to structural aluminum which is close to 2.9 but that is on the high end of the spectrum, I think.  Even measurement error is a possible explanation of the specific gravity+hardness joint discrepancy.

So the real mystery is how this piece came to its current shape.  I've never heard of native aluminum (unlike native forms of metals like copper, silver, gold, and platinum).  Was it intentionally fabricated to look like a rock?  I doubt it.  Was it water worn in a creek/river?  Possibly, but that also seems unlikely.  It wasn't around any other aggregate material.  (I have found lead in gravel backfill before but that's easily explained as hunters' bullets in creeks/streams/rivers when and where the gravel was collected.)  Also, anyone who has detected 45-55 year old ring-and-beavertail pulltab knows that the ground chemicals have done little to degrade those.  Aluminum hasn't been in common usage for much more than a century and a half -- not enough time for the ground to significantly degrade it.

I guess I'll chalk this up to another mystery of detecting.

  • Like 1
Link to comment
Share on other sites

Could be natural tin or zinc, still in it's rock form. Cool find.

  • Like 1
Link to comment
Share on other sites

GB,

   I think the lack of responses means that many of us are not at your level of knowledge, going by all the tests and measurements that you have already done!  I think only a bonafide geologist with the object in hand could identify it properly! Interesting mystery find! Good luck!👍👍

  • Like 1
Link to comment
Share on other sites

16 minutes ago, Joe D. said:

I think the lack of responses means that many of us are not at your level of knowledge,...

I'm just using some methods geologists use -- streak & specific gravity test, and then I've throw in metal detector responses.  Also this subforum is probably not followed to the extent the the main forum and some of the popular subforums are.  And you are right.  Lots of times I see items that people ask for help identifying and I wouldn't know where to begin.

I've not found many (maybe any) hot rocks in my area.  Out west (and in Australia) they are abundant, much to the dismay of detectorists.  A lot of those (what Gerry likes to call 'cold stones') are purely magnetic (i.e. no conductivity).  There are tricks to figure those out.  The bigger problems are the ones that are actually conductive.  They mimic native gold quite well.  I was hoping someone had stumbled across one of those which matched my 'rock', at least in its high detector conductivity readings.  But maybe getting such a high conductance (on both IB/VFL and PI) is so rare for naturally occurring minerals that they just don't show up.  As I mentioned, some native metals (including copper and silver) are found by detectorists.  Obviously mine is way too low in density to be either of those.

  • Like 1
Link to comment
Share on other sites

   Your Chalcophyllite reference, led me to the Cornwall England Tin mining! I just watched a show last night all about the mining there, and the byproduct of arsenic! Which was at first a waste product of tin mining! Later, when they found so many uses for it,  they were the world leading supplier of arsenic! It was used for many "medicinal"  and agricultural products, including womens makeup! Until they found out how toxic and deadly it was to humans and animals! Than used for weapons in WW1! Discontinued before WW2!👍👍

Link to comment
Share on other sites

  • The title was changed to Mystery Rock / Mineral Found While Detecting
16 hours ago, GB_Amateur said:

I was hoping for more responses, and maybe they will trickle in.

Number one rule - need a peek at a fresh surface. Looking at exteriors of weathered rocks or other objects does not give a good enough look at what we are dealing with. Ive been a rockhound for ages, and first thing with anything unknown - break it.

No such thing as native aluminum. But the big question - is it rock or metal? Shatters, or flattens?

But just to look at it, it looks like a lump of weathered serpentine, greenish rock that weathers to reds and browns. Often mineralized, and might signal for being a difference in ground balance between it and background. In other words, a hot rock.

Unless it is metal, then all bets are off.

  • Like 4
Link to comment
Share on other sites

3 hours ago, Steve Herschbach said:

first thing with anything unknown - break it.

Thanks for the reply, Steve, but smash my precious specimen????  :unsure:  I guess in the name of science...  :biggrin:

3 hours ago, Steve Herschbach said:

But just to look at it, it looks like a lump of weathered serpentine, greenish rock that weathers to reds and browns. Often mineralized, and might signal for being a difference in ground balance between it and background. In other words, a hot rock.

I've heard of serpentine -- that it's a PitA for some gold detectorists, including in parts of northern California.  But would it read as a high conductor (>19 dTID) in an air test with the Equinox and also give a strong signal out to 6" on a White's TDI (Miner John 5"x9" folded mono) in an air test with ground balance off, gain of 5/10?

The specific gravity and hardness fit nicely according to this Wikipedia page and it sure has the right appearance, too.  I'll continue to investigate. 

 

 

  • Like 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...