International Gem Society

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What is it?
Donald Clark CSM IMG
President
International Gem Society

     One of the things that has become apparent to me is the importance of practical experience in gem identification. New gemology students often fuss over inconsequential data and miss important clues.
     To rectify this, we have created this series titled “What is it?” The quizzes are designed to be as realistic as possible. They will give you a better idea of how to proceed based on the information at hand. This will not only help you learn the identification process, it will show you time saving short cuts.

Best wishes,
Donald Clark CSM IMG

What is it? #1

     A customer brings you this green stone and wants to know if it is an emerald or a peridot. Peridot is her birth stone, but she know emeralds are much more valuable! With fingers crossed, you begin your examination.

     The first step is to clean the gem and then examine it with a loupe. You find a small feather and note that the stone is very well cut, but nothing more. You take the stone to your microscope and examine thoroughly with dark field and oblique illumination. You see that is has a small conchoidal fracture with vitreous luster on the culet, but you do not find any more inclusions.
     Next you take a basic RI reading; that is two readings at 90 degrees to each other. Your readings are 1.621 and 1.627.
     A quick check in the polariscope shows that the stone is doubly refractive, but you cannot find an optic sign. While examining the stone, you notice that it is at least moderately pleocrotic and colors are yellowish green and green.
     At this point you can begin narrowing down its identity. Check your references and answer the following questions.

Question 1
     At this point, what do you think the stone most likely is?

Question 2
     What test(s) would you run next?

Answers

     Of the above clues; the feather, conchoidal fracture and vitreous luster are of least importance. These are so common as to have little identification value.
     Slightly more important, you know the stone is green and transparent. These features are common to dozens of gems, but they still limit your choices quite a bit. (There is no way it could be malachite, turquoise, opal, etc.) Being well cut usually indicates a gem of some value, as few people would spend much time on a synthetic. This is not proof of anything, but a clue that may be important later.
     The initial tests were chosen because they are the easiest and least time consuming. In just a few minutes you know the approximate RI range, that is is doubly refractive, and moderately to strongly pleocrotic. At this point you can begin going through your reference material to see what the possibilities are.

Question 1
     At this point, what do you think the stone most likely is?

     The answer to this will depend on your reference materials and what data you choose to search for. Here is an easy way: in Gemology Tools, enter the information you now have; (Color, green; Optic sign, DR; RI high and low, (note that we are using just what has been measured, the actual range may be greater); Pleocroism, 2 colors and moderate; Transparency, transparent. The search results will come up with acintolite, topaz, and a few varieties of tourmaline. It is not necessary to identify the variety of tourmaline, so the test comes down to separating acintolite, tourmaline, and topaz.

Question 2
     What test(s) would you run next?

     Topaz has a significantly different specific gravity than the other gems, but specific gravity will not distinguish between acitolite and tourmaline. Each of these has a different optic sign, so that is the deciding factor. You couldn't find the optic sign in your polarisocope, so go back to your refractomer. When you have determined the optic sign, you can make a positive identification.

Post Script

     Tourmalines are one of the easiest gems to identify. Strong pleochroism is a good indicator, especially if you see two entirely different colors, like orange and green. Another quickly found indicator is hollow growth tubes. Either of these factors tell you that it is likely you are looking at a tourmaline. Check the RI next. If it matches tourmaline, then the only other thing you need for proof is the optic sign. No other gem has strong pleocroism, the same RI and optic sign as tourmaline.
     Tourmalines frequently have stress, which makes finding the optic sign in the polariscope easy. However, even if you do not find it there, you can still use your refractometer.

    This is a sample. The rest of our "What is it?" quizes are available to members only.