I honestly didn't realize a gemstone's optic sign could change since I hadn't thought about inclusions, but of course it makes perfect sense. It was informative to me so I thought y'all might like to check it out.
From the International Gem Society:
How Can A Gemstone’s Optic Sign Change?
Question
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?
Regards,
Nitin
Answer
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 ..."
https://www.gemsociety.org/article/how- ... rticlelink
