Persian plan blue is the most desirable colour in turquoise trade. Here is picture of A royal gift - " The Marie-Louise Diadem that has 1,006 old mine cut diamonds with totaling 700 carats . There are 79 turquoise cabochons with total number of540 carats.
This is the most fine and desirable turquoise colour . It should be even, smooth, equal hue saturation, medium intense blue.
This is called a "Persian blue" . It is such splendid colour. Its for 2nd wife of Napoleon. Her name was Empress Marie Louise. It was a wedding gift for her given in 1810. That time however it was set with Emeralds. It's name was " diadem". It remained in the family till it's another owner Van Cleef and Arpels who purchased it in1950s .
Van plucked the Emeralds and sold them individually by setting in jewelry. Replacement of missing stones were done with turquoise. Later on Hairess to the Post cereal fortune, Marjorie Merriweather Post took the turquoise diadem. It was donated to Smithsonian Institution in 1971. It is displayed there right now.