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 Post subject: Natural weathering?
PostPosted: Tue Oct 27, 2015 2:23 am 
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Is it possible for a gemmologist to examine a stone and state to a high degree of certainty that the surface condition of a stone is the result of natural weathering or something else?

The reason for asking is this: A relatively short distance from the Anakie sapphire field, there is a deposit of a material referred to in prospecting guides as an orthoclase feldspar (Springsure labrodorite). I have visited the site several times and have collected some quantity of the material. It is transparent, near-colourless to pale honey or champagne coloured. I can offer no scientific information, I can only say that it is quite light in weight, somewhat softer than quartz when faceted.

Importantly to the question, it comes out of the ground in blocky, angular pieces. Of the hundreds of pieces of material I have collected from up and down the length of this small site (probably several thousand taking small bits into account), not one has a surface that is even slightly smooth. The shape is nothing like that of stream worn stones. This goes for both the material taken from the gravel of the creek bed as well as what is dug from the basalt soil.

I have been given a small parcel of stones marked "Springsure labrodorite", apparently purchased some time ago from a local gem show. All of the stones in the parcel are very smooth with no sharp edges or corners and have the appearance of stream worn stones.

My suspicion is that if the material is indeed what it is supposed to be, then it has been tumbled to make it appear as more aesthically-appealing rough. But if it hasn't been, then that would be interesting to me because it would suggest either another site (I have never heard of one) or a patch of the original site I have not yet come across where the material has a different appearance for some reason.

Is it possible for an examination to reveal whether the smoothing and rounding is likely to be natural or man-made?


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 Post subject: Re: Natural weathering?
PostPosted: Tue Oct 27, 2015 8:30 am 
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I would think that stream worn gravel being in a stream bed of various size grits would have a different finish then a stone that had been tumbled in a tub of uniform grit size....but then if you wanted it to look naturally worn down all you would have to do is add some gravels or grits of different sizes.


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 Post subject: Re: Natural weathering?
PostPosted: Tue Oct 27, 2015 3:24 pm 
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Thanks for your reply Stairman. You don't think it's possible for a gemmo to determine then?

I'm not sure how far the science can take us in identifying stones and their features, hence the question.

If Barbra were to lend me her microscope :) and it revealed to me that one of the stones had a surface fracture containing foreign particles, which upon examination turned out to be particles of silicon carbide, I'd think that would be a dead giveaway.

Failing that, I suppose I'll have to do what we prospecting types do and rely heavily on firsthand field knowledge and experience, information from the senses, intuition and suspicion.


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 Post subject: Re: Natural weathering?
PostPosted: Tue Oct 27, 2015 4:30 pm 
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That would be a tough call, Lefty, even with a scope.
There might be secondary residue present in a fracture which was naturally occurring.
We'd need some serious spectrographic instrumentation to sort out the nature of that residue.
I'm thinking LIBS. I wish I had a Laser-induced breakdown spectrometer unit to loan out....or frankly use anywhere. I don't. :(


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 Post subject: Re: Natural weathering?
PostPosted: Tue Oct 27, 2015 6:18 pm 
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Thanks Barbra. So, a bit of a tough one to crack without some really, really serious equipment?

The material has no particular value although it facets a surprisingly bright stone and I quite like it. I also have a small piece I cabbed which displays a (somewhat weak) phenomenon, a sort of double cats eye, like two gold wires crossing over one another. The stone commonly referred to as Moonstone is also an orthoclase feldspar isn't it?

My thought on the particles was that silicon carbide is commonly used as a tumbling abrasive and that - correct me if I'm wrong here - is entirely mad-made and has no naturally occurring counterpart? Therefore, particles of it embedded in the stone surface - in conjuction with the on-the-ground observation as to the outward appearance of the material in it's natural environment - could be taken as pretty reasonable suspicion that it had been through a tumbler.

Laser-induced breakdown spectrometer - is this the device that is used to vaporize a small piece of a specimen so that the gases released can be chemically analysed?

Cheers


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 Post subject: Re: Natural weathering?
PostPosted: Tue Oct 27, 2015 6:21 pm 
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I know we can not id material from pictures. This question could probably not be answered with pictures, but I would love to see some pictures of each of the materials. If nothing else I am curious.

It sounds like you have a few pieces of it.

There are a few people here that could probably make a very educated guess if they were to be sent a sample. Maybe in exchange for keeping the sample. 8) I am certainly not qualified. But I would be interested in hearing what they say if they see a piece.


According to Gemstones of the world, felspar has perfect cleavage. Maybe the pieces you found in the stream broke along the clevage planes when tumbeling along the stream bed, instead of getting worn.


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 Post subject: Re: Natural weathering?
PostPosted: Tue Oct 27, 2015 6:27 pm 
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Lefty wrote:
The stone commonly referred to as Moonstone is also an orthoclase feldspar isn't it?

Yes, and cat's eye moonstones are not too uncommon. Stars are a bit scarcer but exist.

Lefty wrote:
My thought on the particles was that silicon carbide is commonly used as a tumbling abrasive and that - correct me if I'm wrong here - is entirely mad-made and has no naturally occurring counterpart?

Close. It does occur naturally but it's rare, usually found in meteorites or very rarely in some rocks which have been brought up from extreme depths.

My guess would be that it was tumbled, probably to make any adularescence easier to see (it is usually parallel to cleavage planes which means it's sometimes hard to tell reflections off the cleavage from actual heen). That's not to say you can't find rounded feldspars in nature--it is quite cleavey but it can happen. It may just be from another area.

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 Post subject: Re: Natural weathering?
PostPosted: Tue Oct 27, 2015 10:42 pm 
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Quite cleavy in the tumbler too, I suspect.
What is the luster like on this stuff? A picture would be cool.


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 Post subject: Re: Natural weathering?
PostPosted: Tue Oct 27, 2015 10:50 pm 
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Thanks guys. I would love to post pictures but my computer is really on it's last legs and I can't upload any, nor can I use this one for that purpose. Hope to be getting a new one within a few weeks. I'll see if I can scrounge up some photos I might have posted elsewhere.

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According to Gemstones of the world, felspar has perfect cleavage. Maybe the pieces you found in the stream broke along the clevage planes when tumbeling along the stream bed, instead of getting worn.


Could be Wilson. But the stuff looks the same whether it's in the gravel of the dry stream bed or whether you dig it from the heavy basalt soil. I found a couple of small pieces of stone which closely resemble something marketed in North Queensland as "Quinkan gravel", which is used in the horticulture industry (a very good medium for potting tropical orchids apparently) which is supposedly a form of basaltic scoria. They had bits of the feldspar embedded in them, the same blocky shape.

The cleavage is quite obvious to look at in plenty of the pieces. Actually, the stone is somewhat tougher than I was expecting given it's place on the Knoop scale and while I have only gotten around to faceting a few bits, the cleavage doesn't seem to have posed any significant problem pre-forming, cutting or polishing. I just pre-formed it on sintered #360, cut the facets in on a cheapie plated #3000 and polished with #100 000 on a Matrix. I think I did try polishing with cerium but it was a bit prone to agglomeration scratching if I recall correctly. Critical angle is supposed to be 41 degrees, I just cut the last one in a standard round brilliant with horizontally split mains with a culet angle of 43 degrees.

Anyway, I'll see if I can scrape up one of my old photos of it.


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 Post subject: Re: Natural weathering?
PostPosted: Thu Nov 05, 2015 11:05 pm 
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Currently working on my fourth piece of the feldspar. The third and fourth pieces (I have started the fourth while the third is in the transfer jig) have a feature I did not see in the first two stones, which are complete.

The third showed what looks like "sergeant stripes" colour zoning in sapphire but completely colourless. It was easily visible through the frosted surface pre-polish but became almost invisible on polishing. It is just barely visible through the side of the stone. It has to be the cleavage plane I'm looking at but there is no suggestion of a crack or parting.

The fourth stone shows this feature more pronounced than the third, with the planes easily visible through the side. Again, it does not appear to be an actual crack. It displays as a whitish sheen when viewed at certain angles. When looked straight down upon through what will be the table, it vanishes completely and does not seem to affect clarity.

Whether these last two stones will have their optical properties affected by this and end up looking different to the first two which do not display this feature is a wait and see, hopefully will finish one tonight so I can tell.

I very recently sent some stones away that appeared to be clean enough to facet, I don't know if any of them will display this feature. More experience is needed working with this stuff I think. I also found it tough and slow to polish in one direction but very fast and easy in the other.


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