What is the ‘diamond of the first water’?

In Bridgerton, it is the job of Queen Charlotte to name a debutante as the pick of the social season, a so-called ‘diamond of the first water’.

While the phrase ‘diamond of the first water’ was never used to single out any one young woman who would top the season, it is true that certain women were marked by the press of the day as great beauties. Their names would appear in newspapers, they would be written about and publicly celebrated, becoming celebrities of the season; on occasion, they were ranked, with scoresheets.