Prized for its brilliant color, pastel was first used in the sixteenth century, notably by Leonardo da Vinci. It flourished in the eighteenth century, again in the late nineteenth century, and remains popular today. The optical vibrancy of a work in pastel results both from the absence of yellowing resins in its composition and the reflection of light from the innumerable facets of the finely ground powder.

"Quote for the Metropolitan Museum"

Under a microscope a particle of pastel is diamond like, and reflects light in the same way as a prism, this can be seen when different light hits the painting.