Anomalous birefringence?

This word is mentioned in part 4 on synthetic gemstones,but no explanation is given of its meaning.Also the words natural analogs occurs in part 5.Is there a glossary of technical terms somewhere on this website?Or can anyone enlighten me as to their meaning?I find it very frustrating trying to read articles where certain words aren’t explained!

Hi if you go on Google you can find an array of explanations on Anomalous birefringence as per my attachment