What causes the color in many blue sapphires through intervalence charge transfer?

The presence of iron and titanium in blue sapphires is responsible for the characteristic color through a process known as intervalence charge transfer. In this context, intervalence charge transfer refers to the movement of electrons between different metal ions in the crystal structure of the gemstone.

When iron (Fe) and titanium (Ti) are present in specific oxidation states, they can create conditions that allow for this electron transfer to occur. The absorption of certain wavelengths of light, a result of this charge transfer, leads to the appearance of blue in the sapphire. Specifically, the interactions between the iron and titanium ions produce the necessary energy levels that facilitate this charge transfer process, which imparts the desired blue coloration to the sapphire.

Understanding this relationship is crucial as it highlights the importance of chemical composition in determining the color of gemstones, particularly how specific elements interact within the crystal lattice to create fascinating visual effects.