Which trace elements cause the blue color in corundum?

The blue color in corundum, which is known as sapphire when it takes on this hue, is primarily attributed to the presence of iron and titanium as trace elements. When these two elements are incorporated into the crystal structure of corundum, they interact with the crystal lattice in a way that allows for the absorption of certain wavelengths of light. Specifically, iron and titanium work together to absorb light in the yellow and red parts of the spectrum while allowing blue wavelengths to be reflected and transmitted, which is perceived as the characteristic blue color of sapphire.

This phenomenon is a result of the complex electronic transitions within the iron and titanium ions, which create the distinct optical effects responsible for the color. Other trace elements can impart different colors to corundum, but when focusing on the blue varieties, iron and titanium are indeed the key contributors.