It is an adjective.
Meaning : like or of an opal displaying a fire play of colours.
Opalescence actually is a type of dichroism seen in highly dispersed systems which have little opacity . The material appears blue in the scattered light perpendicular to the transmitted light and yellowish-red in transmitted light. This phenomenon is named after the appearance of opals.
Synonyms : multi-coloured, many-hued, prismatic, rainbow-like, kaleidoscopic, iridescent, lustrous, shimmering,
opalescence - noun
opalescently - adverb
1)The fire works lit up the night sky with opalescent hues.
2)We could see large numbers of divers slipping daily into the Red Sea's opalescent waters.
3)Add small amount of blue to produce deeper, richer, opalescent effects in the picture.