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 Post subject: Hollow-backed cabochons for dark material
PostPosted: Fri Sep 01, 2017 6:48 pm 
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Location: Central Queensland, Australia
As suggested to me by Barbra, I had a go at hollowing out the back of an extremely dark pyrope garnet cab, now that I have acquired a flex shaft and heap of bits.

I used a tiny diamond ball burr (I think it's called), continually dipped in WD40 to lubricate and prevent overheating (hopefully). I was surprised and pleased that it was not nearly as difficult as I was imaging. I made quite a neat-shaped depression in the back with the burr and then smoothed it out with a little conical grinding bit made from either silicon carbide or corundum.

It didn't take that long at all and I was quite happy with the result. I still wouldn;t set this particular stone because I selected a really super dark one (I case I stuffed it up), this stone was black even with strong backlighting with colour only visible around the edges. Once I had finished, the stone was dramatically lighter and easily seen through with backlighting. If a virtually black stone lightens this much, I think the normal too-dark red ones should come up a treat. I also have some too-dark midnight blue sapphires that might look good done like this.

Have I done this the right way? I was going to finish up with a little 3000 grit on a hard felt tip and then either alumina or 60 000.

How long can I expect the burrs to last? (Obviously sapphire will wear them out quicker than garnet).


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 Post subject: Re: Hollow-backed cabochons for dark material
PostPosted: Fri Sep 01, 2017 10:51 pm 
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Sounds like an awful lot of work for a gem that doesn't have a lot of market value, Lefty. I can understand doing it for a challenging cutting exercise and I'm not knocking your effort.

I think machine-cut German carbuncles can be found at attractive prices and they use much lighter-hued rough to begin with. (At least I assume they're machine-cut based on prices). Hope you'll post an image when you're done.

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 Post subject: Re: Hollow-backed cabochons for dark material
PostPosted: Sat Sep 02, 2017 3:31 am 
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Done purely for my personal satisfaction Rick - I have access to a property with a deposit of pyrope garnets on it and while I enjoy finding them, many are too dark for faceting. I have no need of selling to justify the time spent (and the time taken to do this was surprisingly very quick). Here's a way of using the over-dark stones I have collected up over the years.

I like the idea of having finished jewellery piece that I have been involved in every step of the way - from digging the stone from the ground to faceting/cabbing it to making a setting and putting it in. That thought really does it for me.

I'm sure you can buy such stones for pennies but if I bought something then to me, in some ways it's just another off-the-shelf component. Most of the stones in my collection have story that I remember every time I look at them. I have stones that could be sold for a tidy sum and stones that are worth next to no money at all but all are worth the same to me.

So while I might actually make some significant $$$ if I'm very lucky while chasing sapphires, I still chase amethyst, garnets, topaz, labradorite and anything else I can find at every opportunity - even though they can be bought cheaply and for no physical exertion or risk of snakebite etc, it's not quite the same to me.

Prospecting type people get sentimental about our rocks :)

If I get good at drilling out these little stones I might even be able to apply for a job as a dentist 8)


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 Post subject: Re: Hollow-backed cabochons for dark material
PostPosted: Sun Sep 03, 2017 12:11 pm 
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cutting 2 crowns works too, or just a flat base with a shallow crown.


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 Post subject: Re: Hollow-backed cabochons for dark material
PostPosted: Sun Sep 03, 2017 3:59 pm 
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Lefty, I fully understand what you're doing. I often do similar projects for the same reasons you mention.

You're lucky to have such an abundance of gem types to prospect for. I've done a lot of prospecting and field gem-collecting myself and particularly understand your feelings about personally processing a stone you found yourself through every step into finished jewelry. I've done it myself many times.

As for finishing he stone's cavity a lot depends on the grit sizes you chose for your initial grinding and smoothing. If you have access to diamond burs you might try a final smoothing with a 600-grit bur, making sure all grinding marks are removed, then proceed with finer diamond compounds.

I make my carving burs out of wood doweling by cutting off an appropriately-sized piece, drilling a hole in the end and epoxying in a shank salvaged from a worn-out bur (after cutting off the head portion obviously).

Then I shape the wood however I want to with my Foredom handpiece, rotating it against a file, then sandpaper. If you do this you'll need to make similar burs for each diamond grit size you plan to use. I use liquid silicone for lubricant while sanding but many people use olive oil. Final polishing can be done many ways but lots of people use felt burs with diamond.

Have fun and show us the final result.

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 Post subject: Re: Hollow-backed cabochons for dark material
PostPosted: Sun Sep 03, 2017 5:58 pm 
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Cheers Rick.

Seems a bit strange to get attached to a rock but there are plenty who do. Other people only part with what they have to in order to eat. The old guy on the neighbouring claim when I was a kid accumulated a small fortune in sapphires. Jars and jars and jars full of beautiful stones, my parents witnessed them go into a bank vault here in our home town. He could have easily used them to fund a comfortable retirement somewhere nice - instead, he is still living in a tin shack out there on the field (now having people care for him as he is in his 80's) in the hot, dry, dusty bush. A man fascinated by sapphires but with little care for anything else material in the world. Same goes for the guy that took the compass bearing for my ill-fated venture into my own mining claim - he showed me a stunning 107 carat sapphire with the full colour change property - light green in one direction, bright orange in the other and pink in artificial light. Guy still lives out there in the bush, digging in the dirt.

I have a sapphire (still haven't gotten round to faceting it yet) that I would never sell for the reason that it is the first sapphire I found where none are officially known to exist - hundreds of miles away from the Anakie-Rubyvale sapphire field. It will only facet a little 1 carat stone I reckon and I have better ones but those ones all come from a place where they've been mining them for over a century. To my knowledge, I'm the only person to have found sapphire in this particular place. So it's kind of special.

Thanks for the tip, I have a conical felt bit that came with the set but I might keep it for something else and try the dowel.

Quote:
cutting 2 crowns works too, or just a flat base with a shallow crown.


Hi Sean.

Yes, I've down the flat base with a shallow rose cut crown and it was reasonably effective. I will also try the two crowns as you suggest.


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 Post subject: Re: Hollow-backed cabochons for dark material
PostPosted: Tue Oct 17, 2017 12:55 pm 
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I'd love to see photos!

For a typical diamond burr, I'd suggest using a water drip to control heat and dust, surely less trouble than oils. Though for the diamond-impregnated wooden bits, of course you're back to oil.

Try backing the stone with silver foil, best light reflector you can get.

Or get really clever and slip a wee LED into the hollow...

Thanks for your progress report, doing something with the over-saturated garnets we find by the kilo here in Canada has been on my list of things to do for years, maybe now I'll get around to cutting some!


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