Post subject: Why are sapphires glassy while spinels are sparkly?
Posted: Wed Jan 28, 2015 10:13 pm
Established Member
Joined: Fri Aug 03, 2012 3:26 pm Posts: 16
Why is it that clean, light colored sapphires are glassy, while light colored spinels are sparkly? I looked at the characteristics of the 2 species on the The Gemology Project, but am still perplexed. Can a sapphire look sparkly like a spinel?
Honestly, I don't think they are. The big potential difference would be that sapphires are doubly refractive, which can tend to soften the look of back facets because they're slightly doubled, while spinel is singly refractive. But that's going to be awfully subtle. But look at a well-cut white sapphire. Those suckers are brilliant.
Post subject: Re: Why are sapphires glassy while spinels are sparkly?
Posted: Thu Jan 29, 2015 12:05 am
Gold Member
Joined: Wed Nov 15, 2006 9:44 pm Posts: 1079 Location: Washington State
Sapphire and spinel are so close in their eye visible optical properties, that if they are cut the same, they will look the same and it's not possible to distinguish which is which. This, of course, requires the same lighting and none of that great gemological insight provided by "Visual Optics". just a frontal view in the same light.
Could you explain "glassy" a little better? I've honestly never seen a precision cut sapphire that was not sparkly, so I'm a little confused. Part of your observations may be due to a poorly designed and/or cut stone, in which case anything can have large tilt windows or show too much head reflection and look kind of dead.
Post subject: Re: Why are sapphires glassy while spinels are sparkly?
Posted: Thu Jan 29, 2015 8:02 pm
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Joined: Sun Jul 25, 2010 8:34 pm Posts: 222 Location: Far-northern California
I agree that, technically speaking, they should both be sparkly if well-cut. I was surprised however when I purchased my first spinel, at how sparkly it was! I suppose I haven't really seen many light-colored sapphires in person to sufficiently compare.
(If someone wants to send me a light gray, well-cut sapphire, I'll gladly take notes and let you know what I observe.)
Post subject: Re: Why are sapphires glassy while spinels are sparkly?
Posted: Fri Jan 30, 2015 3:01 pm
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Joined: Fri Aug 03, 2012 3:26 pm Posts: 16
Thanks, everyone. Perhaps my issue is that I have seen better cut light colored spinels versus sapphires?
By glassy, I mean that they don't sparkle.
I am always amazed at how sparkly light colored spinels are, but my favorite is precision cut, so perhaps that is the difference. I always end up returning light colored sapphires (light blue) because they don't sparkle to me, but perhaps that is because none have been precision cut. (They are not silky, they just don't sparkle.)
That would have to be bad cutting. Perhaps it is related to spinel's crystal form biasing overseas weight retaining cuts towards better shapes? Odd though, because both places I've had stones cut overseas excelled at sapphire in particular.
Post subject: Re: Why are sapphires glassy while spinels are sparkly?
Posted: Thu Feb 12, 2015 4:04 am
Active Member
Joined: Sun Feb 01, 2015 9:57 pm Posts: 66
Yeah, they are some pretty sparkly beauties. I really like the rhapsody cut. All those gems have quite a lot of facets as well. The only thing I didn't like is the rhapsody cut was a beaute to my eyes, but look at the inconsistency between three different rhapsody cuts.
Look at the tables of
#430 #481 #521
There is quite a drastic change in the shape of the table on all three.
I don't know anything about the cut, but other pictures show round table shape, so maybe there are some variations of the cut, or freestyled modifications.
Anywho, this is a great thread, and I'll have to put some of this to memory.
Post subject: Re: Why are sapphires glassy while spinels are sparkly?
Posted: Fri Feb 13, 2015 12:05 am
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Joined: Fri Jun 25, 2010 10:06 pm Posts: 2267 Location: Chapel Hill, NC / Toronto, ON
Yup, the overwhelming difference is in the cutting. Sapphires, especially larger ones, come in weird shapes in the rough, and commercial cutters will give them odd bellies and weird off-center culets to save weight. Spinels come in better shapes for yield, and it just so happens that cutting them for yield gives them really good proportions for brilliance.
Post subject: Re: Why are sapphires glassy while spinels are sparkly?
Posted: Sun Feb 15, 2015 6:28 am
Active Member
Joined: Sun Feb 01, 2015 9:57 pm Posts: 66
Ahh, I see Arya, thanks. As I know very little of faceting, and am just beginning to understand the basics, that is a help. It's good to know and I appreciate it. Now I know changing things such as culet location, and girdles can trickle all the way down to the table, and every facet on the way there. Likely even the cutting angle of every facet downstream, from my speculation. Until I simulate the whole faceting process, or can comprehend a entire gemstone cut, I will not really know which will affect which. Such as girdle "naturals" trickling down to the table. Or whether offcentering the culet, but keeping the girdle proportional to cutting diagram would do the same, trickleing all the way to the table. I suppose there are many factors, including:
-The order of pavilion, girdle and crown cut, and transfers.
-whether the offcenter of the culet took place at dop transfer, or if the culet proportions were manipulated(angles of 1 or more facets changed from cutting diagram proportions)
-whether such "natural" manipulations were cut meet point, or without meet point using a system of over-riding the natural or a combination of. (I don't know currently whether that is possible, to start a meet-point system of faceting, after somewhat overriding meetpoints for a "natural".)
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