What word, originally applied to a large, flawless diamond, is now used to denote a model of excellence of any kind?

Question

What word, originally applied to a large, flawless diamond, is now used to denote a model of excellence or perfection of any kind?

Answer 1

Accidental
In music, an accidental is a note of a pitch (or pitch class) that is not a member of the scale or mode indicated by the most recently applied key signature. In musical notation, the sharp (♯), flat (♭), and natural (♮) symbols, among others, mark such notes—and those symbols are also called accidentals.