Why do sapphires darken when cut?

[Lefty]

Had a sapphire I dug a while ago, showed it to a cutter who said that there is some good crystal in it but only enough to facet a very small stone (ie, a Lamberts cutter). So I didn't worry about it. But last night I decided "what the hell - I'll cab the thing".

Ended up being about 7mm in circumference, a bit of silk in there. In the rough and early in the cutting process, it was a beautiful shade of blue-green. But once finished and polished up, only stong backlighting would show any colour - had it been put in a setting it would just have looked black.

It was blue with a green cross so prone to being dark I guess, but neither the blue nor the green were an overly dark shade. The green cross was actually a nicer colour I thought but I couldn't orient it that way or there would have been virtually nothing left because of the shape.

I've noticed that dark garnets and coloured quartz seem to actually lighten a bit when cabbed. What is it with sapphire?

 

[fossickeract]

Hi Lefty,
You raise a topic I find very interesting although from a faceted stone point of view, blue /green sapphires often turn out dark or black which can be understood as the (main) c axis only allows blue light to pass and then the (cross) a,b axes block it as they only allow green to pass, most stone cuts even cabs suffer from this issue. Maybe a cut suitable for tourmaline treating the blue axis like a closed c axes and putting the table on the green might give a pleasing result never had suitable rough to try it.
Never know with sapphires how they will turn out till they come finished off the dop. I cut a small clear piece of crystal which only had a spot of blue on the end of the dog tooth, put it in the point of the pavilion, from the side the result was nearly colourless but from above its royal blue? Had a little yellow from Macpherson's creek with a pink/grey cross and it cut a very bright yellow with the odd pink flash. Blue/yellow parties often turn out an undesirable olive green but yellow /green parties are my favourite to cut giving pleasant greens with yellow flashes.
Saw a 15c sapphire found at Grabben Gullen last year with deep red main axis but yellow/green cross now that would be a challenge.

This blue was a dogtooth with only a blue spot at the tip of the crystal.

This was a yellow with a pinkish grey cross.
So with sapphire you have got the main c axis and 3 cross axes each of which can affect the final colour, and darken the stone.
Ted

 

[rough2cut]

I think you deceive your selves when you look at how great a colour is when you hold it up to the light or sit it on a torch lense. If you hold it at waist level with a light shinning down on it then that's what you get. Its the main reason I don't like cutting sapphires. there are very few that will live up to peoples expectations, and when the cutting fee is higher than the stones value it makes you look like a rippoff artist for not knowing how the stone would turn out. This is why I believe stones should be heat treated to make them the best they can be before cutting. If you look at all the posts of rough you can see the cuttable ones easily when there light blue sitting on a mirror, but you can see that they look hazy and need heat treating. No such problem with zircon or topaz.

 

[Lefty]

Nice stones there Ted

Yes, it is a bit of a shame - so many sapphires look magnificent when backlit but dissapoint when finished, especially blue with a green cross.

That said, I have seen some that have finished up stunning stones that show great colour and clarity without needing any kind of backlighting.

And that's what makes a gemstone a gemstone - they are bloody hard to find and really good ones are even harder to find.

Interesting thing about the garnet, it did the opposite to sapphire - it was black without backlighting but once cabbed the red showed up nicely without needing light behind it. Different minerals I guess.

 

[Lefty]

BTW, I know what you're saying rough2cut but a fair bit depends on the aspect they are displayed from. I'm sure that most of us here can appreciate the beauty of a faceted stone all by itself - I know I can at least, I've got some I'll probably never set because I'm content to admire them as they are in their little display cases - but there is really only one purpose to faceting a gemstone and that is to have it set in a jewellery piece.

Hold the sapphire at waist height as you say with light coming down on it - this is about what you see when it's set in a ring.

Hold it between your forefinger and thumb at chest height with your middle finger behind to block the light - this is about what you'll see when it's set in a pendant.

Now hold it up to the light the way you would when you first dug the bit of rough from the ground, but a bit lower at around head height - this is about what you'll see when it's in a drop earring (unless it's very dark). How it looks here will further depend on variables such as the height of the wearer relative to the height of the veiwer, how the person wears their hair etc.

After all, if a stone is teal-coloured then it is teal-coloured, there just needs to be enough light entering it to show it. If you have a matching pair of darker stones, use them for drop earrings and keep the lightest and brightest for rings (most people seem to want a sapphire ring for some reason.)

Obviously, earrings require a matching pair, which is probably an option that you're rarely presented with since you cut other people's stones. It's a bit different when you dig your own, I have a few calibrated pairs of sapphires there.

I agree that zircons and topaz don't usually suffer from the problem you identify but there is something else they usually don't suffer from that sapphires do - a high resale value Shame - I like both stones. Topaz is "poor man's diamond" and looks great imo and I love zircons, so much fire. Some people at the club have scoffed but I really like the beer bottle coloured ones, I think it's a beautiful colour.


A great deal depends upon the point of veiw from which you are considering something

 

[Twapster]

This topic rings home true with my green sapphire found at GG, in the rough it was sensational 7carat and prob would have fetched a good price as it had a great colour under light, almost yellow, once cabbed it turned milky and lost a bit of appeal turning a jade green. I should have had it heat treated!

 

[rough2cut]

You feel bad when someones so pleased with what they have found after they slaved all weekend in a leech infested muddy creek, only to find its not a good cutter.
I hate telling them otherwise because there is always someone to tell them its great to keep up the enthusiasism. 'I like the congac diamonds myself over the white ones.
Diamonds over zircon any day. Its like an Alexandrite ring I had to verify for a family friend and they were adamant it was the real thing. I had to tell them it wasn't and was a synthetic alexandrite coulour change corundum. They had classed it as the prize inheritance piece of the family. So do you tell them the harsh truth and become an ex family friend or keep the tale going.Dark sapphire is dark sapphire and it doesn't matter about the cross section. Dark green or dark blue or inbetween. Its still dark.
Sorry I still hate cutting sapphires.

 

[Twapster]

Im still toying with getting my stone heat treated but not sure where to send it for that, I would just love to know if it clears the silk. Its still 4 carat so if it clears up has potential. Would just be a good experiment to see a before and after...ive seen other stones no where near as good come out ridiculously clear so am curious about the process.

 

[Lefty]

Diamonds over zircon any day.

If you can afford them Actually, there's a bloke who has taken out a lease to prospect for diamonds where I got those garnets. Apparently he's said that people can have the garnets, he's only interested in the diamonds - bet he bars access if he finds something worthwhile though There might be the odd alluvial one there but generally they're too deep for hand mining. The garnets are over top of them close to the surface.

Sorry I still hate cutting sapphires.

As long as people like buying them I'm quite happy

You must get a lot of crappy ones brought to you rough2cut? Yes, the smalls and darks and silk-ridden ones well outnumber the good ones but I still see plenty of goodies turn up on the field (though I'm sure most of the best ones go straight overseas to be re-labeled as having come from a different country of origin). Dad bought one the other day through a friend, it's 7 carat finished, a sqaure cut (not sure what you would call the style of cut) - a beautiful bright green. No backlighting or holding it up needed, just open the case and look down on it and you can see right through it. Came from Rubyvale.

If you want to cut plenty of good sapphire you'd probably have to relocate to one of the fields where you might have a chance of catching a few before they are snapped up by Thai buyers and shipped over there, to be re-labled "fine Thai" or some crap like that. The buying up of Australia's best by foreigners has always been the case really - it used to be Europeans (Germans mainly) nowdays it's the Thais.

BTW, are there any special challenges in cutting diamond? Do they wear your laps quicker?

 

[rough2cut]

Totally different gear and no it doesn't wear the lap any more than a coloured stone cutter would and I can cut a 6mm brillant cut in about 1 hour and fifteen minutes.

 

[blayke]

twapster, same with mine, it is a blue/green cross but I have seen similar rough turn into really nice deep stones that when the sun hits them the blue fires up gorgeous. with the silk in mine I hope it comes out when cooked but if not....well its still my first 50ct cutter, I don't care if its $50,000 or $500 value its still my first big one and ill love it no matter what

 

[Twapster]

twapster, same with mine, it is a blue/green cross but I have seen similar rough turn into really nice deep stones that when the sun hits them the blue fires up gorgeous. with the silk in mine I hope it comes out when cooked but if not....well its still my first 50ct cutter, I don't care if its $50,000 or $500 value its still my first big one and ill love it no matter what

Let me know where u get it cooked! Is it expensive?

 

[blayke]

twapster, same with mine, it is a blue/green cross but I have seen similar rough turn into really nice deep stones that when the sun hits them the blue fires up gorgeous. with the silk in mine I hope it comes out when cooked but if not....well its still my first 50ct cutter, I don't care if its $50,000 or $500 value its still my first big one and ill love it no matter what

Let me know where u get it cooked! Is it expensive?

have been told to get it cooked on the fields you find it, generally more experience with stones of the area. i know that Peter Brown cooks on the Gemfields up north, Lefty may know a bit more but thats where ill get mine done. not sure of cost but apparently he does them in batches so would be cheaper than just getting and individual stone done....

i believe peter owns the gem gallery in rubyvale

http://www.rubyvalegemgallery.com/

will let you know when i get it done

 

[Lefty]

Been a long time since we had any cooked Blayke. Bill Hagan was still doing it back then - to tell the truth, I'm not even sure if he's still alive. He must be about 90 if he is.

 

[Twapster]

Tks for the info as not much about on the process or where to get it done. I'm just curious as to the effect and would be happy to post before and after pics for everyone to have a look.