Sneaky Synthetic Sapphire

Posted on Mar 22, 2017 in Gemcutting, Gemology, Journal, Q&A | No Comments
Sneaky Synthetic Sapphire

Synthetic Sapphire Rough fracture

One of my clients purchased a fine selection of larger sized sapphire rough from Umba, Tanzania. The stones were all nicely shaped and great colors. But in reviewing the stones for cutting, I noticed one that didn’t seem quite right. It seemed to be a water-worn sapphire cobble, but the fracture pattern on one side struck me as very unusual for Umba.

This fracture surface looks like ribs or maybe a washboard. It’s possible for sapphire to fracture this way, but I’ve never seen it in natural rough, so I began to look more closely. Then it dawned on me that the rough had a distinct curved profile. Natural sapphire rough may come in a water-worn cobble that suggests an oval, but this curve suggested a quarter circle; I started to think about synthetic sapphire boules.

When synthetic sapphire is produced by the flame-fusion method, it is grown as cigar-like shapes called a boule. Because of the way the synthetic crystal is formed, these boules are necessarily split in half length-wise to relieve internal stress. This suspect rough was beginning to look a lot like a boule section. Consider these two images: the suspect rough is on the left and a polished cross-section of flame-fusion synthetic sapphire is on the right.

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At this point, it was clear to me that this wasn’t an Umba sapphire rough. Had I noticed this at the buying table in Tanzania, I would have both rejected the stone and had a pointed discussion with whoever offered me the piece. But since I was in my shop and had consent of the owner, I was able to window the stone and get a look inside.

A quick check of the refractive index confirmed that the stone is indeed sapphire (synthetic spinel rough looks similar). But my microscope gave the most telling view:

Synthetic Sapphire Rough interior

None of these inclusions look natural. The perfectly spherical bubbles are characteristic of glass and low-grade synthetics. The arced spangle of blue dots oriented in lines has no natural analog I’m aware of. Additionally, those spangles occur at a distinct color-change boundary (hard to see in the image, the left portion of the rough is near-colorless, the right is deep blue). I would expect this kind of effect near the end of a synthetic sapphire boule when the manufacturer is starting or stopping whatever color doppant is used to create the blue color in otherwise colorless synthetic sapphire.

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The synthetic origin of this stone is now obvious, but it certainly wasn’t at first. Someone took a good bit of time to make this low-grade synthetic look close enough to a natural Umba sapphire to pass inspection in a parcel. In fact, whoever did this even went so far as to add fake dirt to make it look like the rough was fresh from the ground
(hard to show in a picture, but in person and using a 10x loupe it’s obvious this material was glued into place).

Just for comparison sake, here is an image of the typical inclusions you would see in natural Umba sapphire rough:

Inclusions in Natural Umba Sapphire Rough