What are Rubies and Sapphires chemically known as?

Rubies and sapphires are both gem-quality varieties of the mineral corundum, whose chemical composition is crystalline aluminium oxide, Al2O3 [1]. The red color of ruby arises from chromium (Cr3+) substituting for some aluminium in the corundum lattice, while other trace impurities produce the various colors seen in sapphires; but the host mineral in both cases remains Al2O3 [1]. The chemical formula Al2O3 is consistent with aluminium’s oxidation to form aluminium oxide (e.g., 4Al + 3O2 → 2Al2O3), confirming that aluminium oxide is the correct chemical identity for corundum-based gemstones [2]. Hence option 2 (Aluminium oxide) is correct.

Sources

1. [1] https://www.gemstone.org/science-of-stones-ruby
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > Activity 3.9 > p. 41