• Calcite, Laser and Double Refraction

    14 April 2016

    Calcite, Laser and Double Refraction

    From the ISG Journey Thru Gemology of 12 May. Please note that the use of a laser for gemological demonstrations is very dangerous. I do not recommend that anyone attempt to duplicate this demonstration unless you like to live very, very dangerously. You can be blinded if you do this wrong so please be advised. Rj

    Calcite. At left you see a calcite rhomb crystal. This one is often referred to as "optical calcite" since it is very clear, and allows light to pass through it very easily and with little or no interference from structural anomalies.

    Calcite has a unique optical property carried to the extreme. It can take a single beam of light and split it into two very distinct and very demonstrable light beams. What goes on inside this calcite crystal is the same thing that goes on inside double refractive gemstones, taking a beam of light and splitting it into two.

    In the case of colorless calcite, the beams are colorless. But in the case of colored gemstones, the beams can be of a variety of colors, which give us the amazing gemstones that provide us with 2 or more colors based on the light beams being split. Allow me to show this a bit more clearly below.

    From our former kids gem and mineral museum (that was destroyed by the drunk driver) we can see what happens when you read through our large optical calcite crystal. The letters are split and separated into two, demonstrating how this amazing creation of nature offers this wonderful optical property.

    But let's use a laser and see if we can really understand the difference between a "Double Refractive" gemstone (DR) and a "Single Refractive" gemstone (SR).

    By definition, when we point our laser through the DR stone we should get two laser beams traveling through the stone, while a SR stone should only have one beam. Below left you see a laser traveling through our calcite crystal, and below right you see the laser traveling through a SR cubic zirconia which indeed has only one beam due to the single refractive nature of this gem material. The red anomaly at the lower right of this image is due to a crystal malformation that interacted with the laser beam.

    So we have a DR stone below left. Now what if these two beams each had a different color, and they were being produced inside a gemstone, such as a tanzanite? In that case we could use a London Dichroscope to see each beam, and therefore each color. See the next panel set for the results.

    Below left is a London Dichroscope from the Gem-A. And below right is our tanzanite. The London Dichroscope has two polarizing filters (remember our Cheap Sunglass Polariscope last week) which create plane polarized light where each allows only one beam, or color, to pass through. By viewing our tanzanite through a London Dichroscope we are able to view each beam going through the tanzanite, and this allows us to view the two colors of the beams of our tanzanite below right. One is purple, one is violet. Which, of course, is why tanzanite is so remarkable.

     

    You should always remember that the property of double refraction has optical directions that show results more pronounced when viewed from certain direction, and less when viewed from others. Always remember that optically, a gemstone is a 360 degree sphere.

    As a result of this you must view the gemstone from many directions to insure you are getting the best view of the double refractive reaction.

    At left is a slide set of this tanzanite in a view of maximum color separation, and a view of minimal color separation. As this demonstrates, you must take some time and view the gemstone from many directions to get the best results.

    There is also a tool called the Calcite Dichroscope that uses the very property shown above to create the view of plane polarized light on a smaller scale. Viewing through the calcite dichroscope the double refractive andalusite seen below right shows an unusual combination of colors. Two beams of light offering two distinct colors, and we are able to view them using some very basic gemological tools. But what about some practical applications beyond simple gem identification? Well, there is a very, very dramatic example of this very thing seen next.

    Many of you have heard of the famous copper-bearing Oregon Sunstone that has such amazing gemological and optical properties. You may have also heard that there is supposed to be mines in Tibet and the Congo that are also copper-bearing feldspar of the same type as the Oregon Sunstone.

    With our simple dichroscope demonstration, we can prove that the Tibet Andesine and Congo Andesine are.....hoaxes. Fakes. Frauds!

    Simply compare the two views below. Below left you see a true Oregon Sunstone from the Double Eagle #16 mine. Below right is a specimen of "Tibet Andesine" that we obtained from one of the sellers who tout this material as supposedly being natural. With just a cursory look you can see the clearly defined double refraction colors of the Oregon Sunstone. But the "Tibet andesine" color are due to artificially induced coloring elements. Artificially induced coloring elements of this type have no gemological properties, they just have colors. This is exactly what you see in this fraudulent Tibet or Congo andesine being sold throughout the world markets....supposedly as authentic. It is all a fake and a simple dichroscope proves the issue. (although years of litigation involving the ISG were also required to bring this situation to light)

    This gemology thing....its not rocket science. It is not some magical, mystical knowledge that has to cost tens of thousands of dollars to get a famous name on your diploma. It is simply understanding the optical properties of nature's creations, and learning how to see and identify these properties from one gemstone to the next.

    Pointing a laser into an optical calcite crystal is a simple thing on its face. But what you see by this simple demonstration is part of the amazing journey into the study of gemstones.

    We call it the ISG Journey Thru Gemology.

    We invite you to join the ISG and share this Journey with us.

    Robert James FGA, GG
    President, International School of Gemology