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 Post subject: How to polish/cut star sapphire for beginners?
PostPosted: Sat Jun 13, 2020 11:31 pm 
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So I just bought a pound of (potential) star sapphire rough off of Etsy, and I was wondering if there were any easy ways for an absolute beginner to get the star to show itself. From what I looked up, asterism presents itself in corundum on the c-axis (optical axis). Some of these roughs are visibly in the hexagonal shape, which (as far as I know) would make initially finding the c-axis easy, as it should be perpendicular to the hexagonal flat face. However, given how much literature appears to be dedicated to finding the c-axis of a sapphire to be cut, I'm not so certain.

Certain websites also mention that the potential star sapphire could be cut into the shape of a ball or sphere, where the star would then show up on 2 sides of the ball and could be identified for further polishing. In that light, would a potential approach be to tumble the sapphire (with sufficiently hard media, for a very long time)?

I'm aware the best answer is probably to go get it polished by someone who knows what they're doing, but I imagine that isn't cheap and wanted to know if there were alternatives.


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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Sun Jun 14, 2020 11:27 pm 
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Yes the C axis would be looking down like looking at a bolt head.

I have bag of black sapphire said to be star as well but not all will produce a well formed star.
I typically take a few to the arbor and round or oval shape the A-B axis to size large enough to provide a relatively high dome cab. Do not flatten the bottom yet, some room is need to adjust a star to the top of the stone if needed. Start doming the top just like cabbing once your at say 360 or so shine a bright pen light straight down and find the star if there is one. You can keep the stone wet or some mineral oil when doing the light check. If you can get light to pass through the C axis from the bottom and you can see any silk that is a good candidate for a possible star.

I suggest you get to the arbor and just go at few. Light hitting the silk forms the asterism, the silk runs some what parallel with the AB axis in star material, but each piece of rough will normally need orientated.

Hope that helps get you going.
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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Mon Jun 15, 2020 12:14 am 
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elementcollector1 wrote:
Certain websites also mention that the potential star sapphire could be cut into the shape of a ball or sphere, where the star would then show up on 2 sides of the ball and could be identified for further polishing.

I've seen this suggested before, and honestly I think it's a ridiculously impractical idea. It's mostly suggested for rose quartz but spotting the star in those is not a real challenge.

On to the sapphire. Some of them will do better with a low dome than a high dome, unintuitively. Some will refuse to polish on a normal cabbing setup. They can likely be saved by polishing with loose grit on a medium like wood, or can be chucked out the nearest window.
I have found this type of star rough to be a real pain to cut, but the results can be very fun.

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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Mon Jun 15, 2020 9:34 am 
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This is what grooved wooden laps/wheels, and sapphire cups were made for.

Is the rough you bought the black Australian material?


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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Mon Jun 15, 2020 3:11 pm 
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Yes polishing this material is another beast altogether. I use a set of hard maple cups in various grit sizes from 600 to 14000 diamond, very similar to the old Graves copper Sapphire cups that you could buy from graves but these mount into a drill chuck or similar type, I use on my omf machine. after the 14000k stage I moved to a crystallite felt pad glued to an aluminum Master lap with 50000 spray diamond. A very time intensive process .
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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Mon Jun 15, 2020 3:41 pm 
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I am guessing these are Indian stones. I have a few pounds of them myself.
Some of them polish up fine on a normal cabbing machine, or at least good enough. Some pit and take a very irregular polish, or unpolish at higher grits. Don't take it personally if they do.
I didn't have the same issue with Australian sapphires (outside of some of the really flaky 'bomb' sapphires).

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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Mon Jun 15, 2020 10:46 pm 
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https://www.etsy.com/listing/770170320/ ... h-crystals

They're apparently Brazilian, actually. The main picture looks steel blue (and opaque?), but a customer posted a picture with some much nicer light blue ones that look translucent (a little hard to tell in the lighting).

I should have also mentioned that I don't have any official cabbing setup or arbor. There's a diamond saw at work and I have a dremel with a diamond grinding wheel attached (something like 250 grit), but that's the current extent of my gem cutting resources.

If more is needed, then I'd probably be better served reaching out to someone in the US to cut it for me - any recommendations? I'm in Indiana, and wouldn't mind shipping it out as long as it's within the country.


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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Mon Oct 12, 2020 11:03 pm 
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So just to be totally obnoxious, let me verify I’ve understood properly.. the star, generally, would be oriented perpendicular to the c-axis??

I just spent about 8 hours yesterday grinding and polishing away at a promising purple sapphire with lots of visible chatoyance on my crystalite crystalmaster 6” lap with diamond smoothing disks and followed by hi-tech felt polishing pads charged with diamond paste - 8000, 14000, 50000. It now has a glass finish and a “target” (concentric circles) visible on either end (presumably the c-axis). Polishing away at the cursed thing trying to find the star, and if I’m understanding correctly I actually need to high-dome the SIDE (perpendicular to the targets at either end).

Well, this is good to know and I’ll try that next. I’d already cut this stone down to a marble, so it shouldn’t be too hard to dome a side instead!

Thanks for this info! :D


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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Thu Oct 15, 2020 4:01 pm 
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KbneurobotX wrote:
I just spent about 8 hours yesterday grinding and polishing away at a promising purple sapphire with lots of visible chatoyance on my crystalite crystalmaster 6” lap with diamond smoothing disks and followed by hi-tech felt polishing pads charged with diamond paste - 8000, 14000, 50000. It now has a glass finish and a “target” (concentric circles) visible on either end (presumably the c-axis). Polishing away at the cursed thing trying to find the star, and if I’m understanding correctly I actually need to high-dome the SIDE (perpendicular to the targets at either end).
Thanks for this info! :D

Chatoyance does not mean it will positively produce a star of asterism. It may just produce a slight Cat's-Eye. Is that better? 8)

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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Thu Oct 15, 2020 5:40 pm 
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You may have been tricked by one of the three directions of parting these can have. These will typically be sheeny with aligned rose channels (previously called boehmite needles) and maybe other inclusions but won't star. Can you post a picture of what you got?

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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Fri Oct 16, 2020 2:09 am 
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KbneurobotX wrote:
So just to be totally obnoxious, let me verify I’ve understood properly.. the star, generally, would be oriented perpendicular to the c-axis? It now has a glass finish and a “target” (concentric circles) visible on either end (presumably the c-axis).

If there is a star it will be centered on the c-axis. The 'target' effect may be due to parting on the basal plane, that is, perpendicular to the c-axis.


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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Sun Oct 18, 2020 3:49 pm 
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Stephen Challener wrote:
You may have been tricked by one of the three directions of parting these can have. These will typically be sheeny with aligned rose channels (previously called boehmite needles) and maybe other inclusions but won't star. Can you post a picture of what you got?

Stephen, could you discuss "rose channels" in more detail? I bought a parcel of attractive pink corundum with the notion of making some cabochons but ran into big difficulties with distinct parting planes that seem to run in 3 directions, making the cabs unappealing. I researched boehmite needles which I thought were the culprit but I've not run into the term "rose channels." I'm curious.

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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Sun Oct 18, 2020 5:02 pm 
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Whst we thought were boehmite needles are actually empty voids. It's a crystallization feature which was better known in metallurgy.
The GIA has a great video about it:
https://youtu.be/KKwcdk-mXUU
I don't know for sure that they're the cause of parting along those planes, but I wouldn't bet against it either.

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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Sun Oct 18, 2020 5:12 pm 
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Stephen, thanks for the link. I'm hearing impaired so rely on captions. They aren't so great in the video with references to "Burma needles" and "Bromide needles." For a laugh try the captions. After viewing it I think I understand a bit more than I did previously.

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 Post subject: Re: How to polish/cut star sapphire for beginners?
PostPosted: Sun Oct 18, 2020 5:26 pm 
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That's a shame--they should be manually captioning things. Maybe it had community captions before they removed those.
There is a text summary about halfway down this page https://www.gia.edu/gems-gemology/fall- ... nes-pearls

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