Rocks with highly metallic core

[navieko 2]

Hi

Went out detecting with my SDC 2300 at a random dried creek looking for new gold locations. Didn't find any gold, but I found these 2 little "rocks" which sound off like they're gold or lead. You can see in the picture with the 5c coin (Australian) they're relatively small, but pack some weight for their size. The larger one on the left weighs 2.43g and the smaller one on the right weighed .84g.

I thought I'd hold a flame to the smaller one and see if it quickly melts to rule out lead, and after a few seconds a tiny bit of "crust" popped right off to expose a very metallic/silvery looking metal underneath the crust... I'm assuming the bigger one will be the same inside (they were found within a several meters of each other and apart from size look/feel identical). They're not attracted to magnet. The outer crust is quite hard and doesn't seem to scratch easily.

I didn't want to do anything else to them until I have an idea of what they might be. I figure though if I were to keep heating it up the rest of the crust would pop off and I'd be left with just a silver/platinum looking nugget?

Any ideas??

[chrisski 821]

If you have something to measure the volume with, like a graduated cylinder would be good to put water in and then add the samples. 

Gold and platinum weigh 18 to 20 times the water it displaces, so a platinum nugget the size of what you have would displace about .15 ml of water, but since the platinum won't be pure, it could displace more.  Just by the looks of it compared to the coin, my guess is it would not be dense enough for a precious metal.  Silver has basically the same density as lead, 11, but would tend to turn black or green over time.  Silver is also about 50 cents US per gram, and platinum about $25.

Any other tests would be an XRF analysis at say a pawn shop, or acid tests, both of which I have not done.

Edited August 18, 2019 by chrisski

[navieko 2]

Hey thanks for the reply!

I did think of doing a specific gravity test but figured the results would be highly skewed due to the specimen being comprised of at least two distinctly different materials. If I was to completely extract the metal from within the crust/rock then for sure it'd be worth doing the density test.

Another bloke from another forum thinks the outer crust looks to be formed by a process called Iron Concretion which has been found (though rarely) to "encapsulate" foreign objects such as spark plugs or old tools, as well as native objects (other rocks or in this case hopefully a native metal nugget). He reckons to keep it as is for now and get it tested as it's far more interesting & scientifically valuable as is.

Am looking to find a local specialist that can help do some tests and get to the  bottom of it so we'll see... Pretty excited to find out what it is!

Edited August 19, 2019 by navieko

[Au Seeker 3,137]

If you can find a local jeweler or a pawn shop that has a spectrometer they can tell you what the metal is.

[chrisski 821]

10 hours ago, navieko said:

Hey thanks for the reply!

I did think of doing a specific gravity test but figured the results would be highly skewed due to the specimen being comprised of at least two distinctly different materials. If I was to completely extract the metal from within the crust/rock then for sure it'd be worth doing the density test.

Another bloke from another forum thinks the outer crust looks to be formed by a process called Iron Concretion which has been found (though rarely) to "encapsulate" foreign objects such as spark plugs or old tools, as well as native objects (other rocks or in this case hopefully a native metal nugget). He reckons to keep it as is for now and get it tested as it's far more interesting & scientifically valuable as is.

Am looking to find a local specialist that can help do some tests and get to the  bottom of it so we'll see... Pretty excited to find out what it is!

I did not see the concretions in the pics.  The local concretions here are supposed to bubble with a few drops of vinegar.  These concretions are supposed to be a limestone or calcite.  Maybe a drop of vinegar will look.