I have a Presidium Gem Tester and my Alexanderite tests on the Spinel line, but is no Alexanderite listed on the marker. Is that the correct group or is there another reading I should be reading? Thank you
Hi Julie! Welcome to the community!
I did some research on the Presidium Gem Tester series.
Spinel and Chrysoberyl (Alexandrite) share similar thermal conductivity properties. Further digging found some scientific articles that agree.
Chrysoberyl and Spinel overlap in thermal conductivity indicated with the thermal conductivity comparison chart from Presidium here.
Your instrument could be telling you the correct reading, as long as you have properly calibrated it.
A specific gravity test could help distinguish natural alexandrite from natural spinel. Here is a good simple setup on YouTube.
Testing for birefringence will help distinguish the two as well since Alexandrite has birefringence where Spinel does not.
Cavate: There are some color-changing Spinel and I am not familiar with their optical properties. Hopefully someone with better knowledge will chime in here.
Here are two resources from gemdat.org for Alexandrite and Spinel. Similar data sheets can be found here at the IGS site.
Hope this helps.
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Hello Troy,
I appreciate your follow-up and response! This has been a tremendous help.
I will download the Presidium chart, which I was not able to find, but thanks to you.
I now have access to it.
All the best,
Julie
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There are many synthetic color change corundums that’ have been unfortunately referred to as Alexandrite for about 100 years. The color change is from a blue to a more amethyst-like purple, nothing like the color change of a natural Alexandrite. Not to be confused with actual synthetic Alexandrite, this is a simulated Alexandrite at best. Ironically, they are often set in 18K gold, making the stone appear more important than it is. Nice stones, but relatively worthless as they are common and cheap. There is an actual synthetic Alexandrite available, but only in the last decade or two.
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I tested it on both the Presidium Gem tester, which tests as a real stone, and the Presidium PRMI, which had a reading of 1.740, which puts it on the Chysoberyl spectrum as an actual Alexanderite.
Hi Julie,
The IGS Refractive Index list indicates Spinel can also have a R.I. = 1.74; therefore, it cannot be ruled out at this point.
The PRI-M from Presidium is an interesting instrument. It can measure the indicated R.I. of a stone. But it has some limitations as it is difficult to measure the birefringence/optical sign response. Other instruments such as a Polariscope, or a critical angle Refractometer will be needed to measure the optical sign.
It will be necessary to determine if your stone is birefringent or not to potentially separate it from the Spinel family.
Cheers!
I would like to add that an RI alone is not sufficient for identifying stones, you must look at several different factors including but not limited to: Color, inclusions, quality of cut, whether it is singly or doubly refractive, specific gravity, RI, etc… The RI will help you narrow down the possibility of what stones it could be, but you really need a microscope to look at the inclusions to determine growth patterns, this will also help distinguish natural vs synthetic. You should look at the quality of the cut, was it cut to maintain the weight of the stone, or just to be nice and uniform. So many factors play into the identification of gemstones, and what I have stated is by no means an exhaustive list. Hope this helps.
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Thank you.