Alumina is the common name for the ingredient aluminum oxide. It is a naturally occurring mineral used as an abrasive, thickener and absorbent in cosmetics.
Alumina is sometimes used as part of a coating mixture for nano-sized mineral UV filters such as titanium dioxide. This coating keeps the very fine power particles of the mineral sunscreen on the surface of the skin, as the coating materials have a larger molecular size. These coating mixtures also improve the aesthetics and uniform application of mineral sunscreen ingredients.
In skin care preparations, aluminum oxide is used in amounts up to 25%. It has been determined safe for use in cosmetics by the independent Cosmetic Ingredient Review Panel. It is also approved by the US Food and Drug Administration as a color additive for use in certain over-the-counter drugs.
Aluminum oxide in its natural form is a white solid. It is the third hardest naturally occurring substance and is the primary constituent of rubies, emeralds and sapphires (their color is determined by small impurities found in heavy metals). https://www.paulaschoice.com/ingredient-dictionary/ingredient-alumina.html?q=Alumina&fdid=ingredients