I’ve found that many gemstones have their optic sign listed in reference works as both B+ and B- (biaxial positive and biaxial negative). How is optic sign determined and why does this happen? What does it actually mean when the stone’s refractive index (RI) is checked with a refractometer?
Keep in mind that a gem’s optic sign is determined by which RI varies the most, the higher or lower. This is a handy way of categorizing gems but it’s not an unchangeable characteristic. Due to impurities, most minerals have some variation in RI. Even the RI that varies the most can change. Axinite is a classic example. Its optic sign is usually biaxial negative. However, as the amount of magnesium in its chemical composition increases, the gem becomes biaxial positive.
In other cases, the change in optic sign is a result of which axes were measured for the listing. Remember that biaxial gems have three optic axes. Take alexandrite, for example. Its alpha axes range from 1.740 to 1.759, (a range of .019). Its beta axes range from 1.747 to 1.764, (a range of .017). Its gamma axes range from 1.745 to 1.770, (a range of .025).
When taking an RI reading, you only measure two of the three axes. Most of the time, the gamma axis has the lowest RI and the greatest variation. This makes the stone optically negative.
However, it’s possible to measure just the alpha and beta axes, in which case the higher RI will vary the most. It’s also possible for the gamma axes to be higher than the other measured indices. In that case, the stone’s optic sign will also become positive.
Donald Clark, CSM IMG