Post subject: Synthetic Spinel Rough Quality/Cracks?
Posted: Wed Oct 12, 2022 10:06 pm
Established Member
Joined: Sun Feb 09, 2020 10:52 pm Posts: 43
Hi Everyone! It's been a while since I've posted anything here (although I've been reading a lot recently). Perhaps a few of you may have some insight into some synthetic spinel I recently purchased to cut.
I have not purchased or cut a lot of synthetic Spinel before, but the pieces I have had have all been pretty clean. It seems that some cracks near the tip of the boule are not uncommon, but the latest batch I received seems to have a lot more cracks than I'm used to seeing!
Perhaps my expectations are just a little too high... Have I just been lucky before and this is typical for this material? or is this material indeed below average? I have over a kg of it, and I'm debating if I should return it or just cut it and see if I can work around the flaws(I'm used to doing that with Natural rough!)
It looks like a lot of the cracks are shallow or running parallel to the surface creating a thin layer of cracked material along the outside of the boule.... but many of the cracks seem to run almost completely through the boule. It looks almost like I'd end up with split boules like most corundum if I tried to use many of these ones. What do you think?
Post subject: Re: Synthetic Spinel Rough Quality/Cracks?
Posted: Wed Oct 12, 2022 10:56 pm
Gold Member
Joined: Tue Apr 05, 2016 7:58 pm Posts: 1424 Location: San Marcos, CA
Definitely looks to be stressed material. If it were mine I would slice one down the a/b axis @.25, .50, .75 creating 4 pcs and doing a forensic inspection looking down the c-axis to see how deep the major stress lines go.
Post subject: Re: Synthetic Spinel Rough Quality/Cracks?
Posted: Wed Oct 12, 2022 11:39 pm
Moderator
Joined: Fri Jun 25, 2010 10:06 pm Posts: 2267 Location: Chapel Hill, NC / Toronto, ON
Hey Mike! As one of our resident experts in flame-fusion synthetics I'm happy to contribute
Historically, flame-fusion spinel has a large amount of internal strain. Part of that is the doping agents used, and part of that is strain generated as the molten material crystallizes onto the seed. With modern production techniques, groups like Djeva, Union Carbide, Linde Co., and the like were able to nearly completely eliminate strain.
Unfortunately, Djeva went bankrupt and a lot of these cheaper Chinese manufacturers just do mass-production without good temperature controls and environmental controls. Or, they just go for production speed since they're totally fine to have boules fracture - they'll just use the bits for smaller stones. So - your expectations of intact boules without these major cracks is exactly what you should expect. It's just that modern cheap growers screw it up.
From your photos, it looks like you have a LOT of internal strain. Typically, that strain is at its worst closest to the seed crystal (the tapered end). So, here's what I'd recommend. Find the lowest point in the boule (meaning closest to the tapered end) where there are no cracks that extend deep into the body of the stone. Saw through there, so you have one side that has a bare minimum of surface crazing, and one side that has some deep cracks. Then, saw through the largest deep crack and see how the strain unfolds. Typically, what'll happen is that as soon as you touch the saw to the crack, the spinel will split cleanly.
Once you've split everything, do some preforming. Grind off all of the surface crazing, then let the boule "rest" on a countertop somewhere and see if any new cracks develop. If not? Then you're good!
Post subject: Re: Synthetic Spinel Rough Quality/Cracks?
Posted: Thu Oct 13, 2022 12:11 am
Established Member
Joined: Sun Feb 09, 2020 10:52 pm Posts: 43
Thanks for the quick input! I do believe the previous material I had was from Djeva.... maybe that's why it seemed way better!
If this is the "new normal" now though,... I'll see what I can do with it.
My first thought was to grind off the surface crazing before going to the saw. I figured it's a 50/50 shot to see which option prevents major cracks from growing. I had parcel of tourmaline years ago that needed to be done like that... but it sounds like you've been down this road already, so I'll try the saw first and see how it goes. Hopefully I don't have a bunch of self destructing stones!
It's too bad that that I'll have a much lower yeild than planned, but that can sometimes be the nature of buying rough based on a few photos(that don't show the severity of the flaws).
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