Designs for a Blue with green cross table Sapphire

[drft_er]

Gday All,
I have a piece of rough with green cross table to try and cut a stone out of for a friend. I have cut Round brilliants before but i am looking at other designs  to get a better return. It has cracks that I have to work around so I am still stuck in planning stage. The C axis and shape of the stone means it will naturally be a rectangle or similar. Has anyone got any ideas on designs to get good colour or light return or that have worked for them?. I am cutting a Garnet (dark stone) in a RUBICELLO cut by Marco Voltini as an experiment to see what sort of result it gives. Any suggestions or designs would be appreciated.
Cheers

[drft_er]

Ill' post a photo tomorrow once i have time
 

[Lefty]

I've tried a few different designs with dark stones - mostly garnets - and not had all that much success. A dark stone will be a dark stone for the most part. I have had some partial success getting some very dark blood red pyrope garnets to return some colour by cutting them in a standard round brilliant with a simple pavilion consisting of only one tier of facets (the breaks) cut at 37 degrees. With a critical angle of about 34.5 degrees for garnet, you can get away with that without fisheyeing it. Should be able to do the same for dark sapphire since they have about the same critical angle (but I haven't tried it yet).

I cut the crown in the Lowboy variation of the SRB. This results in a very shallow stone which still appears black in dull light but returns a lot of vivid red flashes in better light. A round shape seem to accept more light than the square or triangle shapes that I've also tried.

I haven't tried doing anything like that with a rectangle but that's been my experience anyway.

Cheers

[Gem Ranger]

Hi Drft_er,

As you might know its often difficult to get good results with natural sapphire. Designs that retain weight often darken colour which is no good for dark rough. Design selection is also dependent on what your customer is willing to accept - i.e. traditional cuts vs something new.
Blues with green cross tables need to be cut within 15 degrees of the blue, otherwise green flashes can appear around the girdle and lessen the value of such stones. An AFG article by Eric Oliver recommends the following for blue/dark green sapphires 'The culet should never be less than 38 degrees. With dark stones it is better to sacrifice weight than to end up with a piece of black coal. Pavilion facets - culet 38 degrees minimum, Mains 40 to 43 degrees (for worst stones); Crown Angles - Mains 35-37 degrees.'
As for designs, I don't have enough experience cutting sapphires to recommend any, but you could try one of the following options;
- select a suitable design from http://www.facetdiagrams.org/database/ based on the L/W of the rough and then test it on Gemray to check for a good light return; or
- purchase either of the following design series https://shop.lightninglap.com/bk-gem-store/60-designs-for-corundum/ or Book 4 in this series www.northcoastlapidary.com/?p=752

Given that your rough is best for a rectangular design and thinking outside the box, you could even try modifying a design for closed axis tourmaline to limit the return of green colour from the cross table. But keep in mind such designs often have deep pavilions which might struggle to return light if the rough is dark.

Either way if the rough is valuable perhaps a test cut in synthetic sapphire first is the way to go.

Cheers,
GR

[drft_er]

Gday GR/Lefty/All
GR The stone belongs to a mate  who used to mine in the 70's. I did not realise this and we were having a chat one day about gold areas around here as his father used to care take the mine at Dittmer near Proserpine in the 50's - 60's after it last closed, one thing led to another. To cut a long story short he started bring packets of rough (rubbish) that he had laying about for me to practice on.Most of it was very included or cracked but i have cut a few nice stones out of it for him. There is no pressure to get a good stone as he knows it isnt top class. The last 2 stars i cut came out of a piece the he was told had to many cracks for the fields cutter to want to play with, the brief was give it a go you stuff it up tough luck. He did offer me one of the stars for cutting them but i refused as he has looked after me in that department before, besides i think it is invaluable experience for me to be able to have these stones to play with. I have attached a doc with some photos of the stone.
Hey Lefty
I have had some experience with those dark garnets same thing kept the stones shallow. Put them on Gemcad and fiddled with the angles unfortunatly unless I decide to go with 2 round which i could get out of this stone. I sort of have licence to experiment as this stone has been knocked back by other cutters as well.


Cheers

[Gem Ranger]

Hi Drft_er,

I don't know for sure how limiting that crack will be but even at 20% return that still a 3ct + stone. The rough appears quite dark but it has nice blue on the c-axis on which it will need to be cut. If you have access to the gemfields, heat treatment prior to cutting can sometimes lighten a dark stone. Treatment used to cost about $1 per carat but I'm sure that's gone up now. The person running the treatment can tell you if its worth it.
If it were my stone I'd stick with a traditional type cut in a standard shape - from what I can see a pear design might be what your after. If that's the case PC05018 Pendeloque would be a good design to try. PM me with your email address if you need it.
If it all works out, such a stone at 3-4cts could be worth as much as $3k, so even if you don't rate the material its still worth giving careful consideration to.
Best of luck with it.

Cheers,
GR

 

[MakkyBrown]

I have had some experience with those dark garnets same thing kept the stones shallow. Put them on Gemcad and fiddled with the angles unfortunatly unless I decide to go with 2 round which i could get out of this stone. I sort of have licence to experiment as this stone has been knocked back by other cutters as well.


Cheers

I've tried going shallow aswell but not really happy with the results. My latest train of thought is going for maximum iso, using a design that delivers 95% ish ISO over the broadest tilt possible.
Cheers
Andrew

[Lefty]

I've tried going shallow aswell but not really happy with the results. My latest train of thought is going for maximum iso, using a design that delivers 95% ish ISO over the broadest tilt possible.
Cheers

Yeah, at the end of the day a dark stone will be a dark stone no matter what you do usually. Still, I was happy enough with a couple of them. With dark garnets I found that holding the whole bag of the rough up to the light reveals that some are actually significantly lighter than others. None of them would pass a white paper test but the ones that looked lighter in the bag still cut stones that returned some nice red flashes in good light.