Magnesium Stearate

The magnesium salt of stearic acid — a fine white powder, and the same ingredient used as a flow agent in tablets and supplements. In cosmetics its role is almost entirely physical. It stops powders caking, adds bulk, and gives pressed products their slip. It is a formulation aid, and there is no meaningful sense in which it treats skin.

The particles are hydrophobic — they repel water — and they are shaped so they slide over one another easily. In a pressed powder that is what stops the product clumping in humidity and what gives it a smooth, silky feel going on. The same water-repelling quality helps a powder stay put rather than absorbing moisture from the skin and becoming patchy. CosIng also lists a moisturising function, which reflects that lipid-derived powders can slow water loss slightly at the surface, though that is a side effect rather than the reason it is used.