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 Post subject: The geology of gem crystal formation and deposition
PostPosted: Tue Aug 23, 2016 3:59 pm 
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Location: Central Queensland, Australia
About time I put a thread up specifically addressing this topic, given the amount of fossicking I do. One replete with lots of nice pictures of hills, gullies, rocks, outcrops, cutaway views of the ground layers etc. The more I can learn about the geology of the natural environment I am chasing rocks around in, the better.

With luck, I should be going again this weekend as usual. I'm going to start with gemmy quartz - yes, boring old quartz crystal, I promise before the fossicking season ends here I will have reported on a few different kinds of gemstone :) The amethyst and smoky crystals are close handy to where I live and my fossicking partners have not been able to travel further afield of late. Also, since they appear to be right where they formed and have not travelled, it's a good opportunity to study the environment in which crystals large enough to be of interest have formed.

This new site - pretty sure we will be allowed to keep going back - is more interesting than the old site a few miles down the road. That one is on flat ground while this one takes in a series of hills from which weathering granite domes are protruding - I actually found a golf ball-sized, shattered amethyst crystal sitting on top of a rock there last weekend, almost as though someone had placed it there - running down into a couple of small rocky creeks at the base. The creek is full of stream-worn quartz crystals, washed down from the dome I expect. It will be interesting to see what else that is heavier than quartz may have become wedged down between the rocks and boulders. It will also be interesting to excavate right up against the domes themselves.

First photo-story by Sunday/Monday evening hopefully.


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 Post subject: Re: The geology of gem crystal formation and deposition
PostPosted: Tue Aug 23, 2016 10:54 pm 
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From the quartz page....

Quote:
Because they are usually the last to form and simply fill out the remaining voids in the rock, you will rarely see ideally-shaped quartz crystals in a granite, while well-formed crystals of feldspar can be found frequently.


Then either the rock called granite in my area is not granite or it is of a rare condition because where it has decomposed into a coarse sand, we find many perfectly-shaped quartz crystals of decent size. I'm yet to see anything I can identify as a feldspar crystal in this area.

Perhaps my crystals formed in veins and other such structures throughout the granite. Well-formed crystals are abundant in this area in what appears to be the remains of horizontal veins, vugs and pipes. I haven't found one such structure intact yet but in this new spot we might just find them given that the rock domes are still there.


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 Post subject: Re: The geology of gem crystal formation and deposition
PostPosted: Wed Aug 24, 2016 2:51 am 
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Faceted two stones for property owners granddaughter, a deep-coloured smoky from that pipe we hit a little while back (Jeff Graham's 'Lighthouse" and a light-coloured amethyst from the new place (Jack Rowland's "Arrow"). Wife delivered them today, she said she was like a kid in a candy store, jumping up and down, squealing - "of course he can back there and dig any time!"

The power of faceting :)

Now imagine what I'll be able to do once I can hand over fully finished jewellery pieces :D


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 Post subject: Re: The geology of gem crystal formation and deposition
PostPosted: Thu Aug 25, 2016 9:27 pm 
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This post is slightly on topic :wink: .

My favorite book on gem formation was written by John Saul. It's called A Geologist Speculates

I bought a copy when it first came out and gave it as holiday gifts.
It is available for free pdf download now.
Is this the best news ever? Yes and no. I think you should buy the book.
http://www.tomebook2.com/geologist.php

The pdf is available through Lotus Gemology. The link is on this page:
http://www.lotusgemology.com/index.php/ ... s-gemology

Hint: One needs very high download speed to make this downloadable pdf a reality.


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 Post subject: Re: The geology of gem crystal formation and deposition
PostPosted: Thu Aug 25, 2016 10:21 pm 
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Thank you muchly Barbra! :D

I will attempt to download that, looks very interesting!


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 Post subject: Re: The geology of gem crystal formation and deposition
PostPosted: Thu Aug 25, 2016 10:29 pm 
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It's just over 63 megabytes. Took less than a minute for me, but from what I've heard about Australian internet it may be longer for you.

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 Post subject: Re: The geology of gem crystal formation and deposition
PostPosted: Thu Aug 25, 2016 10:42 pm 
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I'm on my break at work at the moment so I can't download it here, I'll have to try at home tonight.

Yes, a first-class broadband network was to be put in place by now but it got scuttled by political jostling between the two major parties :( How such linear progression as uncontroversial and logical as going from telegraph to telephone ever became politicised to the point that it's existence was compromised beats me.


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 Post subject: Re: The geology of gem crystal formation and deposition
PostPosted: Thu Aug 25, 2016 10:54 pm 
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My Comcast is not serving me well.
I'm switching to Sonic in Sept. Fingers crossed.

Finally downloaded. Refreshed my memory.....this book is awesome!


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 Post subject: Re: The geology of gem crystal formation and deposition
PostPosted: Fri Aug 26, 2016 12:07 am 
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No offense, but I've had a very quick look and frankly I would not consider him a credible source. Any time you see someone making wide, sweeping conclusions outside their fields of expertise, you almost certainly have trouble. Like, say, a geologist trying to redefine cancer biology (without knowing much biology, apparently--the cambrian explosion isn't the appearance of complex, multicellular life!). Someone with good scientific grounding should have the sense to know better. Even things he should have some grounding in are troublesome; he apparently really likes abiotic models for oil and gas formation, for instance, which have long been rejected (because they're wrong).

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 Post subject: Re: The geology of gem crystal formation and deposition
PostPosted: Fri Aug 26, 2016 5:49 am 
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While I understand what it is in general, I'm still trying to get my head around what does and what does not constitute a pegmatite. From the quartz page...

Quote:
The look of the quartz specimen found in pegmatite pockets very much depends on the chemical composition of the originating magma and the residual fluids. These fluids are very hot and may contain extremely aggressive agents, as hydrogenfluoride, that will readily attack many silicate minerals. Quartz specimen from such pegmatites often show a rough and dull surface, while being clear inside.


This was how we found the smoky crystals from the "pipe-like" structure to be - a rough scaly surface but many of them were very clean and clear inside, good to use as faceting material. However, many of the crystals from the same place have an extremely smooth and shiny surface to the crystal faces, looking as though they just came off a polishing lap. Lots of fluctuations over a very small area obviously.

Another thing common to the old spot and this new one as well as the distinct band of quartz pieces and crystals with pockets and pipes of crystals deeper below, is the presence of a distinct layer of rusty-orange sand with occasional small nodules of what looks like hematite (haven't done a streak test on any bits yet). Many of the people who dig at the old area have noted that the orange sand often holds some of the best, most intensely-coloured amethyst crystals. Given that amethyst owes it's colour partly to some form of iron and irradiation (radon gas from the granite?), I'm guessing that there's a relationship here.


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 Post subject: Re: The geology of gem crystal formation and deposition
PostPosted: Sun Aug 28, 2016 7:32 am 
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Fossicking partner seems to have gone MIA and incommunicado. He wasn't at the property gate and his mobile is out of commission. I thought about driving down to his place but he still lives some distance from the property and by the time I drove down there and back to the nearest gas station, I would have been just about running on fumes (if diesels could run on fumes). He and his dad live in an isolated farmhouse, hope they are ok. Hopefully he'll contact me soon to say everything is ok.

I decided to push on alone, which isn't really a very smart thing to do in the bush. I can't get buried in a hole collapse here because you can't dig all that deep before you hit solid granite.

Didn't take many photos, didn't want to wander too far from the vehicle on my own. This is the beginning of the granite emerging from the hill. The domes and boulders get thicker as you go further up the hill, we haven't been to the top yet. A little further up the slope from here and the ground is basically just granite rock.

Image

A shallow scrape about a foot deep. That's about as far as you can get down (there's likely some deeper spots). What I thought was the top of an orange sand layer was the top of the granite floor, rusty orange in colour.

Image

The only sign on the surface that there might be something there - bits of crystalline quartz embedded in the granite. Fossicking partner broke a fully-formed crystal from out of a boulder last time.

Image

Found hundreds of odds and ends, mostly pale and cracked amethyst and crystal pieces, most were just a few inches under the surface. About half an hour before leaving, I wander a few dozen paces and sank another hole and was straight away rewarded with a pocket of nicely-coloured amethyst crystals. Mostly cracked of course but I'll facet a few stones from them I think. The more intensely-coloured crystals usually seem to be in pockets and pipes, the broad sheet of quartz just under the surface mostly contains very pale ones.

Despite being such a common mineral, I like quartz. I love the look of the crystals as they come from the gound, looking like little plates, building blocks, medieveal castles and cathedrals. I like how that seems to affect polishing the stuff somewhat less :)

Image

Image

Image

It takes quite some time to really get your head around an area. The owners granddaughter says he grandfather has several very large crystals from the property that he uses as doorstops. I believe that in time I will find some good, big specimen clusters as we work our way around the granite boss.

There


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 Post subject: Re: The geology of gem crystal formation and deposition
PostPosted: Sun Aug 28, 2016 11:47 pm 
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Stephen Challener wrote:
No offense, but I've had a very quick look and frankly I would not consider him a credible source. Any time you see someone making wide, sweeping conclusions outside their fields of expertise, you almost certainly have trouble. Like, say, a geologist trying to redefine cancer biology (without knowing much biology, apparently--the cambrian explosion isn't the appearance of complex, multicellular life!). Someone with good scientific grounding should have the sense to know better. Even things he should have some grounding in are troublesome; he apparently really likes abiotic models for oil and gas formation, for instance, which have long been rejected (because they're wrong).


Remember the title of the book.
A Geologist Speculates.

As someone who was raised to challenge the status quo, I find the book compelling.
We have to make sure we don't replace religion with what we are currently taught scientifically.
Who knows? It might be all wrong. We are all to willing to accept incredible theories explaining things which couldn't be explained before

I resonate with the philosophy of Roger Penrose. In a nutshell, the universe and everything within it functions according to a theory we have not discovered yet. The theory of relativity, quantum mechanics, quantum gravity, string theory......how much do we swallow without question?


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