Magnesium stearate is an ester of magnesium & stearic acid (vegetable based). It is a fine, soft white powder that is used as an additive when making eye shadows. It helps micas adhere more uniformly on the skin while giving it a fluffy even consistency.
It is used in concentrations of 3.5%-10% by weight.
Industry professionals claim it works best for pressed eye shadows, foundations and other pressed cosmetics, but works great in a loose powder.