Answer: The optic sign of a biaxial gemstone is determined by taking RI readings of the stone with a refractometer. If the higher readings vary the most, the sign is positive, but if the lower readings vary the most, the sign is negative. If the high and low readings vary equally, the stone is said to be without sign. The 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.
Impurities Can Cause Variations in Optic Sign
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.
Biaxial Gems Have Multiple Optic Axes
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 axis ranges from 1.740 to 1.759, (a range of .019), while its beta axis ranges from 1.747 to 1.764, (a range of .017). Its gamma axis ranges 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