Facet doubling

[Lee Little]

I just got home from Cambodia and had the pleasure of visiting the area that is famous for Blue Zircons. When shopping for a stone that has a high refractive index such a Zircon it is handy to remember to notice the doubling on the back facets as this will make you more sure you are getting a Zircon rather than a simulant. You can see numerous examples of this doubling in this photo. Best regards, Lee

 

[Lee Little]

While in the Zircon market I came across some clean stones that were a little more blue than most Zircons (the picture is not accurate on color but I an showing it as the inclusions are easy to see in it) and upon close examination I could not find any doubling on the back facets in any of them. Finally I found one that was included, inclusions I had never seen in Zircons and they appeared to be possibly gas bubble arrays in straight lines. I bought just that one stone and headed back to my hotel where I had a refractometer and checked the RI. 1.73, synthetic Spinel! That explained the lack of facet doubling as Spinel is a singly refractive gemstone. Best regards, Lee

 

[VapidLapid]

Bear in mind though that if the zircon is cut spot on the optical axis there will be no facet doubling observable through the table. You will then need to look from the side through the crown facets to see the birefringence.

 

[minousbijoux]

Bear in mind though that if the zircon is cut spot on the optical axis there will be no facet doubling observable through the table. You will then need to look from the side through the crown facets to see the birefringence.

VL: can you expand a bit for those of us who are interested but have much to learn?

 

[LD]

Lee you did right (as you found out) those straight lines of bubbles are a complete give-away. Glad you resisted the urge and only bought that one out of curiosity.

 

[chrono]

Absolutely; the colour would first raise suspicion, then the lack of facet doubling, then the type of inclusion. I also look for facet doubling when buying sphenes.

 

[Lee Little]

Bear in mind though that if the zircon is cut spot on the optical axis there will be no facet doubling observable through the table. You will then need to look from the side through the crown facets to see the birefringence.

VL: can you expand a bit for those of us who are interested but have much to learn?

Hi Minousbijoux,
What VL is saying is that you basically tilt the stone and look every direction through the top of the stone and you can see the doubled lines. Just do not expect to see it right through the table like this first picture shows on every stone. This one was the easiest example that I found for show and tell. Best regards, Lee

 

[Lee Little]

Lee you did right (as you found out) those straight lines of bubbles are a complete give-away. Glad you resisted the urge and only bought that one out of curiosity.

Hi LD,
The lighting in the market was very poor and with a 10x loupe I could not see as clearly as the picture shows (taken with a microscopic camera and a fiber optic light) that the lines were composed of bubbles but could only tell that there were chalky straight lines that resembled growth tubes. Generally straight lines, as opposed to curved or wavy, indicate a natural stone. That had me going as I could not figure what else could be a Zircon color (the stone is actually a blue found in Zircon but the color adjustment on the camera was off when I took that picture) and though I suspected synthetic it did not click as a Spinel until I saw the reading. I have never before come across any synthetic with such obvious lines of gas bubble arrays, usually they are only slight and require higher magnification than 10x. I was excited to buy this one just as a study stone and for show and tell. Best regards, Lee

 

[Lee Little]

Absolutely; the colour would first raise suspicion, then the lack of facet doubling, then the type of inclusion. I also look for facet doubling when buying sphenes.

Hi Chrono,
Yes, the color was a bit richer than others there but when I took that picture I did not have the color adjusted so it is not accurate. The environment was a market dedicated to local Zircon, almost no other stones were offered for sale so I was really looking hard to see facet doubling. The lack of it was the first thing that tipped me off that these stones were an exception to the flavor of the market and were likely simply not Zircons. Out of 20 stones in that parcel, this was the only one with any inclusion under 10x so I got lucky with it. You are right, Sphene is also one that always has the doubling. Often with Sphene I just quickly hit it with a pen light, the reaction is virtually unmistakable. Best regards, Lee

 

[Richard M.]

Bear in mind though that if the zircon is cut spot on the optical axis there will be no facet doubling observable through the table. You will then need to look from the side through the crown facets to see the birefringence.

That's especially true in the case of zircon. Cutters often deliberately orient zircon, Moissanite and other strongly birefringent uniaxial stones with the table perpendicular to the optic axis. In the case of zircon it's done to avoid the "fuzzy" face-up appearance caused by back facet doubling. It's done with Moissanite to make it appear singly refractive, like diamond. In both cases stones face more sharply and crisply with that orientation.

Richard M. (Rick Martin)

 

[minousbijoux]

Thank you Lee and Rick, I understand it now.

 

[uber]

Hi Lee,

I saw a gemstone which is being claimed as a blue zirconia (here in Cambodia). The color is the same as your 1st picture on this thread. I did see a straight line of bubbles using a magnifying glass (the square foldable ones jewelers use). Do straight lines mean natural or synthetic?

It's being sold at $80 for 3 (plus something) carats. I love the color but don't know how to tell if it's really a blue zircon or a spinel or another stone altogether.

Can you please expound more on facet doubling? What's the RI of zircon? Maybe I can check if the jeweler can measure it for me.

Hope to hear from you and thanks!!!