Opalescence is a kind of light play that happens with certain high quality gemstones. In these gemstones, light reflects and bounces around the gemstone structures which give them a wonderful aura and sometimes a milky blue form of iridescence.

An optical axis of a gemstone crystal is a direction in which a ray of transmitted light suffers no birefringence (double refraction). An optic axis is a direction rather than a single line, all rays that are parallel to that direction exhibit the same lack of birefringence. Gemstone may have a single optic axis, in which case they are uniaxial, or two different optic axes, in which case they are biaxial.   Non-crystalline materials generally have no birefringence and thus, no optic axis.

Gemstones that are made by or derived from living organisms are known as organic gems.  These include pearls from oysters, amber from tree resin etc.

An issue in AI and machine learning where the model learns to perform well on the training data but fails to generalise to new, unseen data.