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…