The characters in Land of the Lustrous are composed of living crystalline gemstones.

This is Phos, or more precisely, phosphophyllite. They are a hydrated form of zinc phosphate, and forms a greenish-blue transparent crystal.

The hair colors of the characters all correspond to the real-life crystal appearance of the corresponding gemstone. Dia (diamond) is white, Cinnabar (crystal mercury) is red, etc.