Joined: Wed Dec 30, 2009 11:44 am Posts: 525 Location: PAKISTAN-Peshawar
My Dear teacher Barbra Voltaire!
You are right that the birefringence of corundum is low. I accepted that doubling of facet is not dominate in the case of corundum as comapare to tourmaline, Zircon etc but if you see the second picture of the post, you will observe that the backing facets junction looks double. There is a clear distance between the facets junction. Teacher this is my personal opinion.
Joined: Mon May 14, 2007 11:00 am Posts: 1133 Location: Monterey, CA
Barbra Voltaire wrote:
Perhaps..perhaps not. Sapphire has a low birefringence; I wouldn't expect to see doubling of the back facets.
If I may state the obvious to those not trained to use only GIA approved techniques, looking for doubling of the back facets is a technique that is 100 or more years old. Times have changed. Do you hear me, GIA?
Today, one doesn't need a loupe, simply use the Hodgkinson Method, and you can numerically estimate RI, birefringence, and dispersion instantaneously. The very low Birefringence:Dispersion ratio is obvious in sapphire and diagnostic.
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21600 Location: San Francisco
Clarification, please. Are you saying, Dr. Hanneman, that one looks for doubling of back facets as a diagnostic characteristic of sapphire?
Really?
If we are attempting to separate topaz from sapphire, I doubt observation of doubling of the back facets will give us any clues at all.
But to pick up on your other topic, I think folks looking to identify unknowns with minimal investment in tools would be EXTREMELY wise to learn Visual Optics. It's not hard. I remember when I had my first VO party at my house (with some GIA Alumni). We had dinner with candles and everyone, including my son, who was 14 years old at the time was able to make some profound determinations, after about 5 minutes of practice. PS Alan Hodgkinson was a guest at the party....that helped.
Joined: Mon May 14, 2007 11:00 am Posts: 1133 Location: Monterey, CA
We both know one can't identify anything on the basis of one observation. Doubling of the back facets is diagnostic for doubly refractive gemstones.
What I am saying is, Use of the Hodgkinson Method can be diagnostic. The doubling of the back facets, low dispersion, low birefringence, coupled with a Birefringence:Dispersion ratio of less than 0.5, and an RI between 1.7 - 1.8 . All of these properties can be instantly determined using the Hodgkinson Method, and combined they are diagnostic for sapphire—regardless of color.
Now, can you tell me even one advantage to be gained by using a loupe?
Of course, you can separate topaz from sapphire with your eyes closed. Simply squeeze it between your fingers. Topaz will slide out of your grip.
After rereading the above reply, I would like to add. The value of the Birefringence:Dispersion ratio should be taught to all students, even though I promote it. Do you hear me, GIA?
Wow iv been out of town for a few days i wish i had checked this post before now . I had no idea such great gemstone minds would be posting on my little topaz post but this is GO .A friend suggested that i use the LED computer screen and polarized sunglasses thing you know what im talking about and im allmost sure i found out its double refractive But becuase its so clean inside (no inclusions) it was suggested that it synthetic sapphire But i cant see striae So its very confusing to me im not a professional just a gem addict trying to learn All the other gems from this person have checked out no problem so... Can a lab gem have color zoning
Joined: Sat Jul 11, 2009 10:29 am Posts: 928 Location: USA
Yes, color zoning can be in synthetic corundum if that is what it is. Topaz can be extremely clean though, typically, since so common, it is not typically cut unless really clean.
Joined: Mon May 14, 2007 11:00 am Posts: 1133 Location: Monterey, CA
gemstoneJEFF wrote:
I just did the Hodgkinson thing and there was NO green two rainbows and From top to bottom the rainbow was blue white a little yellow then orange
What you are seeing is the result of two overlapping "rainbows" and the result is white. This is charactistic for doubly refractive gems having low birefringence.
While this is more suggestive of corundum, try to estimate the birefringence:dispersion ratio or the RI by how large the circle of images appears, i.e. can you see more than two at a time.
swishman it does make sense for this to be synthetic sapphire but when i look at this stone in the darkfield then a synthetic sapphire they just dont look the same to me there is a very small chip on the faset junction near the botom of the pavillion
bill i just squeezed on it and also took out a irradiated blue topaz to compare the feel and the stone in question was not at all as slippery as the blue topaz i did it alot to and washed and dryed the stones and my hand to make sure and every time the topaz was always more slippery
Joined: Sat Jul 11, 2009 10:29 am Posts: 928 Location: USA
Jeff, There are different types of synthetic corundum, pulled/Czochralski process and Verneuil process/flame fusion for example. They both look different under magnification. .
You may not have syn. corundum at all, but the RI really points to it. And the fact that the stone is not slippery at all compared to known topaz is not a good sign either. P.S. I love that test, great for separating quartz and topaz in a hurry, lol.
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