ISG Report: The Story of Feldspar!
There is far more to the feldspar gemstone group than diffusion treated andesine?
With all of the talk about feldspar recently, I thought we might take a look at the feldspar family this week. Just so we all know that there is more to this gemstone group than this diffused andesine that we all seem to be talking about so much these days.
The feldspar group of gemstones is actually an array of gemstones with many common properties. The basic chemical make up of the group stays the same, but the slight differences in crystal structure and chemical make up cause the feldspar group to offer up many unique varieties.
All are basically aluminum silicates. But with the first group you have what is sometimes referred to as the “K” feldspars, or potassium feldspars, and also known as the orthoclase feldspar group. This is due to the presence of potassium as a main constituent along with the aluminum silicate. However, in what is sometimes called the Na or sodium feldspars the potassium is replaced with sodium and calcium, which joins with the aluminum silicate to form the “Sodium” feldspars, also known as the plagioclase feldspar group.
There are two main members of the potassium feldspar or orthoclase group that you are most likely familiar with, amazonite and moonstone.
Moonstone is known for…well, the moon. The adularescence of moonstone gives the look of a moonlit night due to the optical property of the lamellar structure of the gemstone that reflects light in various patterns. Sometimes it is a sheen that appears to float on top of the stone, and sometimes it forms a cat’s eye moonstone effect due to the linear formation of the lamella inclusions.
Amazonite is named for…..the Amazon River where it was first found. This is mostly used for ornamental jewelry items due to the softness of the stone. But the beautiful blue green color has made it a favorite gemstone for jewelry and ornaments for centuries.
Perhaps the interesting gemological feature of both amazonite and moonstone is that they have very closely related Raman readings. At left is a reading of moonstone feldspar. The amazonite is virtually identical. This will be an important point as we move into the plagioclase feldspar group.
Plagioclase Feldspars Indian Sunstone (oligoclase)
Officially known as oligoclase feldspar, the India sunstone is named for its source and is sometimes called aventurine feldspar. This is due to the presence of platelets of tiny hematite platelets that form up inside the stone and reflect light. The reflected sunlight gives it the common name of sunstone. If you look closely at the image above left you can see some of these platelets in the gemstone on the right hand side of the image. The Raman scan of Indian sunstone is quite different from that of the orthoclase feldspars, making separation of the types quite easy. You can see the Raman scan of the Indian sunstone above right.
Labradorite
Of course no discussion of feldspar would be complete without talking about labradorite and labradorescence. The rough specimen you see at left is known as spectrolite and is most famous from the source in Finland . You notice the rainbow of colors, primarily in the blue to yellow colors that play across the stone. In the image at right you can see a finished cabochon gemstone of labradorite under 20x magnification displaying the very beautiful colors of labradorescence for which this member of the feldspar group is famous.
Oregon Sunstone Labradorite
The King of the Feldspars is the Oregon Sunstone Labradorite. And nobody can cut Oregon Sunstone Labradorite like Marty Guptill of
GemArtGallery. Marty donated this wonderful Lone Star cut from the Dust Devil mine for an auction to raise funds for our Raman Microscope, but we just could not part with it. So we collected funds here in the ISG office to buy it and keep it in the ISG Student Reference Collection. Some things are just too special to let go. And this Lone Star Sunstone by Marty Guptill should stay in the Lone Star State of Texas.
But back to the Oregon Sunstones..... This rare member of the feldspar group is found only in the State of Oregon in the United States , and offers an extremely unique color selection due to the presence of copper in the feldspar crystal structure. This unique combination of plagioclase labradorite and copper has made the Oregon Sunstone Feldspar in great demand world wide. So much that the recent andesine debacle was attempted to cash in on the rarity, importance and value of the Oregon Sunstone Labradorite material.
Of course, there is no way to artificially duplicate the Oregon Sunstone without leaving behind serious footprints of the person doing the diffusion treatment trying to emulate the real thing. But at right you can see the unique Raman footprint of the true Oregon Sunstone Labradorite, and below you can compare this to the imposters.
Andesine
For anyone who has done any real research on true andesine, the pictures that come up on the mineral websites look nothing like the gemstone you see at left. This is a classic red andesine that has been sold on the market since 2002 that no one seems to be able to find the mine that produces it. This stone was sold by Direct Shopping Network (DSN) and sent along with a large collection of other stones to the ISG office for us to test.
Along with this DSN stone we were sent the stone you see at right. This is one of the licensed and laser inscribed 2008 Olympic Andesine gemstones that DSN has been licensed to sell by the US Olympic Committee. You can see the laser inscription in the image at right.
But we believe there may be a problem with this 2008 Olympic Andesine….it may be the same material as the diffused andesine we have written about over the past few months.
Here is why the questions have been raised regarding this 2008 Olympic Andesine. Notice the two images below of gemstones in an immersion cell. At left is the known diffusion treated andesine with the filled lamella ribbon that we have seen over and over in the diffusion treated andesine. Notice the striped and banded coloration of the red in the stone. Now notice the 2008 Olympic Andesine from DSN. It has the same striped color banding structure. This was an interesting issue to us in the ISG lab as all of the diffusion treated andesine (and we have about 100 now) show this feature in the immersion cell. But wait! There’s more!
At this point we have done over 900 Raman scans of this diffusion treated andesine in various forms and formats. And we can pretty well pick out the diffusion treated andesine, even for newbies to the Raman world. We demonstrated last week that all of this diffusion treated andesine has virtually the same Raman footprint. So we compared this 2008 Olympic Andesine’s Raman scan to those from our previous report. You can see the results below with the Raman scan of the 2008 Olympic Andesine in blue and the known diffusion treated andesine in black.
Looks like a pretty good match to us. In fact, in all categories of gemological testing the two specimens of 2008 Olympic Andesine that we have in our office both test out exactly as the other 100+ specimens of diffusion treated andesine in our lab.
However, it should be noted that DSN continues to claim that this material is perfectly natural and untreated in any way (based on their last representation we received).
But my main question to this situation is simply: What might the US Olympic Committee do if they found that DSN had been selling artificially color enhanced andesine under the Olympic Logo, without disclosure as required by the US Federal Trade Commission? Certainly a question being asked around this office.
But that is the story of feldspar, in something of an abridged version. Feldspar is actually a very large group of gemstones that can provide wonderful opportunities for unique and beautiful jewelry items that are both cost effective and long wearing.
Coming Next Week: The Andesine Story in Pictures
You will not want to miss next week's edition. The Andesine Story in Pictures. We have the pictorial history of our investigations that no one has seen to date. These are the images that we have not had the time or space to offer you. They are coming next week in The Andesine Story in Pictures. Don't miss it!
Robert James FGA, GG
President, International School of Gemology
A SPECIAL THANK YOU: The ISG has prepared a special
Hall of Honor Section in the ISG and YourGemologist for all of the good people who helped us obtain the Raman Microscope from Enwave Optronics which made this study possible. Without each one of you this would not have been possible. RJ
©2008 International School of Gemology .
All images are taken using the ISG Student Reference Collection of gemstones in the ISG office. No copying or duplication without permission. We do urge and support sharing of this information in its entirety, with copyright notices intact, to others who are interested in the study of gemology. Jeweler’s Associations are welcome to distribute to your members.
