Historically a very expensive ground lazurite pigment, synthetic pigment has been used since the 1830's making it one of the most popular and affordable blues, Yves Klein blue is mostly Ultramarine. (Series 2) 

 Pigment Classification: Inorganic
Color Index Name: PB29
Color Index Number: 77007
Chemical Description: Polysulfide of Sodium-Alumino-Silicate
Opacity/Transparency: Semi-Transparent
Lightfastness Rating: ASTM 1
Permanency: Not for Exterior Use