Understanding Refractive Index Issues in Fire Opals

Question
When measuring the refractive index on fire opals I get correct values on some stones but on other stones I get no reading at all.  No line on the scale.  This is not an uncommon result for me.  Are they some sort of synthetic?  What's up?

Answer
Hello Ron~

Thanks for an excellent question.  Refractive index is not an easy measurement to take, and most likely if you have not been doing it for a long time there is some part of your technique that might be fine-tuned to ensure better results.  There is no synthetic fire opal that I know of, but it is often simulated with glass and plastic. It is possible that you have a simulant and you can try specific gravity as a means of separating these from fire opal.  You will also need to use magnification to look for tell-tale inclusions like swirl lines and bubbles.  

To be certain of your RI results, make sure you are using the best facet of the stone- that is, the facet with the very best polish.  Make sure that it is clean- bear down on the stone with that best facet against a piece of white notebook paper or typing paper and move it around a bit.  This should remove any dirt or residue.  

You should shake your RI liquid vigorously, then let it settle a bit.  The proper amount of liquid is a matter of experience- but one general guideline is to use about the amount that would be covered by the head of a pin.  Don't hesitate to put your stick down on the refractometer stage two or three times, with space in between, to get progressively smaller dots of liquid.  Put your stone on the smallest dot, then slide it onto the hemicylinder.  

Because you are getting readings on some stones, I am guessing that your head is positioned properly and that you know how to bob your head up and down (with considerable range of motion) in order to find the line.  

In summary, make sure you are using the right amount of RI liquid and that your stone is clean.  Shake your liquid well, let it settle, and put the liquid on the stage and NOT the hemicylinder.  I know these sound like very simple concepts but only a lot of experience will tell you if you are using proper technique.

Best of luck to you Ron and please be persistent.  You will soon get very accurate results with your refractometer nearly 100% of the time.

Take care,
David Fortier, GG
Staff Gemologist
Tiptopgem.com