1. Ruby and sapphire are mainly composed of alumina, so they show different colors because of other impurities. Ruby is red because it contains chromium oxide, while sapphire is blue because it contains iron oxide and titanium oxide.

2. Among the main components of bauxite, the content of alumina is very high.

3. Alumina is the reason why metallic aluminum is not easily corroded in air. Pure aluminum easily reacts with oxygen in the air to form a thin aluminum oxide film covering the aluminum surface exposed to the air. This alumina film can prevent aluminum from being further oxidized. The thickness and performance of this oxide film can be enhanced by a treatment process called anodic treatment (anodic corrosion protection).

Aluminum is a good conductor of electricity and heat. Silicon carbide, the crystalline form of aluminum, is suitable for grinding materials and cutting tools because of its high hardness.

5. Alumina powder is often used as a carrier for chromatographic analysis.