I'm brand new to this site and forum! Seems like a great resource I'm about 1/3 completed the Stone Identification course via GIA Distance Ed. I also design and fabricate my own jewellery.
My question pertains to a fairly large Star Sapphire that I was generously gifted about 8 years ago--I don't know anything about it's previous history or how long it was in the possession of the person who gave it to me, but it's quite lovely. GIA doesn't cover a lot of ground with Star Stones and their treatments so I'm hoping that a more experienced gemologist can help me to determine whether it is natural, synthetic or treated. I'm considering sending it to a lab, but would like to know if I've missed any obvious signs that it's been treated or if it's synthetic before I commit to the rather hefty price tag of a lab report and/or appraisal.
It's definitely not a straight up synthetic from what I can tell. It's proportions are a little unusual but very attractive: approx. 13.4 x 18mm with a height of 10.9mm. It has an attractive, fairly solid deep blue body colour under a penlight (it looks completely dark blue under normal light), with a very defined, strong moving 6 ray star. The surface polish looks great but there are less than 20 very minor scattered angular blemishes that are about 1/100mm on the surface. The base is roughly finished and it's slightly domed with a 2mm squarish cavity on the base. When illuminated from behind, the stone has straight colour banding (blue and yellowish green) at a 60º angle. The yellow banding is semi-translucent when light directly under the stone and appears to be full of silk. Other than that, there are no obvious inclusions noticeable with a 10x loupe.
So far, this seems to be a natural star sapphire from what I've learned--straight colour banding at 60º says so to me (?). But I've read a little about diffusion treatment for star sapphire. Any ways to tell whether the star is natural? Anything else I should be looking for? I have a 10x loupe, dichroscope, polariscope and spectroscope.
Should I bite the bullet and send it to a lab without further speculation?
I'd like to ultimately set this stone in a piece of jewellery for sale. Any suggestions on where I can look on-line (aside from ebay) to get a reputable but rough idea of retail values for natural untreated and diffusion before I have things confirmed at the lab?
Thanks so much for your time. Your thoughts are appreciated.
Hi Caroline, well, with the straight lines you are seeing, and the fact that you can see two, or more colours, you can be sure you have a natural stone. The "star' is caused by rutile inclusions, as you will be aware, so you are going to see rutile, looking like silk veils. The original crystal was hexagonal, so if you are seeing 60degree "corners", you are looking at a fragment of that original crystal. Star sapphires/rubys have, in the past, been the most undervalued of the corundums, but are really starting to be valued. In the rough, the star corundum has a bronze, or silver sheen, and to test if a good star is present, we lap a smooth face, then put a dollop of honey on, which gives a convex surface, and shows up the star asterism. Try entering "The black star of Queensland", in your search engine, for the story of a magnificent stone. How many carats is the stone? Barry
I would like to add that you seem to have a natural base for the star sapphire. Noone can tell you it is not diffusion treated without proper analysis. So though it seems good, if you can not determine it yourself send it to, for instance, AGL to get a report (which would be a good idea anyway).
Joined: Wed Jan 23, 2013 5:29 pm Posts: 1047 Location: Paris
Hi Caroline Wouldn't you post a picture of your stone under the right illumination ?
In star sapphires, some colours are more suspicious than others regarding the possibility of diffusion on natural stones. Especially, in my experience, the dark, nearly opaque, blue ones, much zoned innerly. Also, on the diffused ones, again from my experience, the titanium needles are minute, very difficult to see even with magnification, different from the longer needles you can see in natural stars. The diffused stars are overly neat and well defined.
Of course none of these personal observations can replace a lab report...
The stone is approx. 28.5ct and is definitely a deep, solid blue in normal lighting and a royal blue, with some very slight milky patches under a penlight--I'll try to get some photos up later
On further inspection, I think I'm seeing some colour concentration in a crevice which to me points to diffusion treatment. I think that I will send it to a lab now that I'm pretty sure that the stone does have a natural base.
Joined: Wed Jan 23, 2013 5:29 pm Posts: 1047 Location: Paris
If you don't wish to spend too much money on a lab analysis and if you know someone who cuts stones, you can ask for a gentle repolishing of the top of the dome. The diffused stars are very superficial. If the star is diffused it will go away quite easily !
Hi Isi, you can go and stand in the naughty corner! (please don't be offended) Poor beautiful star, fancy having all its "bling" rubbed off. I would have a really beautiful piece of jewellery made, a pendant with the stone surrounded by diamonds maybe, and to heck with the diffusion. It could become a family heirloom, handed from mother to daughter. Barry
Joined: Wed Jan 23, 2013 5:29 pm Posts: 1047 Location: Paris
I agree Barry ! I am a naughty girl. My suggestion is very destructive, and doesn't suit everybody. I myself owned such a star sapphire, very beautiful but which I strongly suspected to be diffused. I had it repolished but only on the top of the dome. The junction point of the star went off immediately... But I kept the legs intact. I liked it all the more when I knew its true nature. But I didn't like it enough to want to have it worn as a jewel... It went into a pedagogic collection, where it turned out very useful. A good heirloom for a school of gemology. It just depends on what you are after !
Joined: Tue Mar 13, 2007 11:12 am Posts: 484 Location: Chiang Rai Thailand
A picture would help tremendously with my guess work but it sounds like a natural sapphire that has been color enhanced and has a diffused star in it. Not terribly valuable but certainly has a place in the market, one of my best sellers. Best regards, Lee
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