What it is
Alumina is aluminum oxide (Al2O3), a chemical compound of aluminum and oxygen whose natural crystalline form is the mineral corundum. In cosmetics its standout role is as a pigment carrier: it is blended with inorganic sunscreen filters such as titanium dioxide and with colorants, often together with a silicone coating, so the particles disperse evenly and spread uniformly on the skin, which is especially useful in mineral sunscreens and makeup. It also works as an absorbent or mattifying agent, a viscosity controller, an opacifier that reduces a formula's transparency, and an abrasive in exfoliating products. It is not itself a UV filter.
Origin & sourcing
Alumina occurs in nature as the mineral corundum, the same material that, with trace impurities, forms rubies and sapphires. Industrially it is extracted from bauxite ore by the Bayer process, in which the ore is dissolved in sodium hydroxide, aluminum hydroxide is precipitated, and that hydroxide is calcined at high temperature to yield aluminum oxide.
Questions everyone asks
Does alumina block UV rays?
No. It is not a UV filter. In sunscreens it mainly coats and carries the mineral filters and pigments so they spread and disperse evenly.
Is alumina the same as aluminum metal?
No. It is aluminum oxide, a stable mineral compound of aluminum and oxygen, the same material found naturally as corundum, ruby, and sapphire.
Why is it in tone-up and mineral sunscreens?
It helps pigments and mineral filters disperse evenly and can add opacity and a smoother spread, which supports an even cosmetic finish.