The common ones are α-Al2O3 and γ-Al2O3. Corundum in nature is α-Al2O3, hexagonal close-packed crystals. α-Al2O3 has a melting point of 2015±15°C, a density of 3.965g/cm3, and a hardness of 8.8. It is insoluble in water, acid or alkali. γ-Al2O3 is a cubic close-packed crystal, insoluble in water, but soluble in acid and alkali, and is a typical amphoteric oxide.
- The main components of ruby and sapphire are alumina. It shows a different color due to other impurities. Ruby contains chromium oxide and is red, while sapphire contains iron oxide and titanium oxide and is blue.
- In the main component bauxite of bauxite, the content of alumina is the highest. Industrially, bauxite is purified into alumina by the Bayer process, and then converted into aluminum metal by the Hall-Heroult process.
- Alumina is the reason why metal aluminum is not easily corroded in the air. Pure metallic aluminum easily reacts with oxygen in the air to form a thin aluminum oxide film covering the surface of aluminum exposed to the air. This aluminum oxide film can prevent aluminum from being oxidized further. The thickness and properties of this oxide film can be enhanced by a process called anodizing.
- Aluminum is a good conductor of electricity and heat. The crystal form of aluminum emery is suitable for abrasive materials and cutting tools because of its high hardness.
- Alumina powder is often used as a medium for chromatography.
