Contamination makes aluminum oxide a priceless gem

Background

Aluminum oxide or corundum is a commonly available material. However, trace levels of contaminants in the crystal structure of aluminum oxide can result in priceless gems, such as rubies, sapphires, and emeralds. Rubies are corundum with chromium contaminants. It's the chromium that produces the deep red color. Next to diamonds, rubies are the most precious gems. Sapphires come in many different colors: yellow, red, pink, and the more traditional color, blue. The blue color is only possible when titanium and iron are both present in the structure. The gem shown in Figure 1 was labeled a ruby at a gem show.

However, the X-ray diffraction pattern collected on the Rigaku MiniFlex, shown in Figure 2, matches corundum and the two additional phases that can be found in sapphires, mainly, rutile [TiO₂] and cronstedtite [(Fe²⁺)₂Fe³⁺(Si,Fe³⁺O₅)(OH)₄].

This gem would be more appropriately called a red sapphire.