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. 

Pigment Classification: Inorganic

Color Index Name: PB29

Color Index Number: 77007

Chemical Description: Polyulfide of Sodium-Alumino-Silicate

Opacity/Transparency: Semi-Transparent

Lightfastness Rating: I

Permanency: Not for exterior use

Munsell Notation Listing: Hue 8.0 PB Value 2 Chroma 4

Gloss Average: 23.89

Viscosity Range: 400-1000 CPS

CIE L*a*b* Values: L*24.36 a*12.28 b*-25.74

Tint Strength: 79.94