means The quality of shimmering with a milky, shifting play of colors, like the surface of an opal or the inside of a seashell.

from From 'opal,' the gemstone, which traveled into English through Latin 'opalus' and Greek 'opallios,' likely tracing back to the Sanskrit 'upala,' meaning simply 'stone' or 'jewel.' The '-escence' tail is the same Latin suffix found in 'luminescence' and 'fluorescence,' marking a state of becoming or giving off — so the word literally means 'the act of going all opal-like,' catching and scattering light the way the stone does.