Generally speaking, pure, uncontaminated alumina is white.
However, the alumina materials we encounter in daily life can exhibit a variety of colors depending on their crystal form, purity, particle size, and impurities. The following diagram briefly illustrates the relationship between different forms of alumina and their colors:
1. Naturally Occurring Aluminum Oxide (Corundum)
Pure α-Al₂O₃ (single crystal): Colorless and transparent, this is colorless sapphire or white sapphire.
When doped with impurities, it can exhibit a variety of colors, which is why precious gemstones are so popular:
· Chromium: Red, ruby.
· Iron and titanium: Blue, sapphire (Note: Sapphire is not limited to blue; corundum gemstones of other colors are also called sapphires).
· Containing other elements can also produce yellow, green, purple, and other colors.
2. Common Industrial Alumina Raw Materials
Smelting-grade alumina: Typically a loose white powder. Produced from bauxite via the Bayer process, it is the raw material for the electrolytic production of aluminum ingots.
High-purity ultrafine alumina: Also typically a white powder, the whiteness varies depending on the purity. Used in precision ceramics, sapphire crystal growth, and polishing agents.
3. Activated Alumina
A specially treated alumina, typically produced in pellets or blocks, is white or slightly pinkish white. It is primarily used as an adsorbent, desiccant, and catalyst carrier.
Summary
Alumina type | Main forms | Common colors |
natural corundum | Gems/Minerals | Colorless, red, blue, etc. (depending on impurities) |
industrial raw materials | powder | White |
activated alumina | Spherical, blocky | White, slightly pink |
So, when someone asks “What color is alumina?” the safest answer is that it is essentially white, but natural gemstones can appear in a variety of colors due to impurities, and industrial products are usually white.
