Question

\[\text{Why do the }Ti^{4+}\text{ and }Fe^{2+}\text{ ions in }Al_2O_3 \text{ make sapphire blue?}\]http://www.geolite.com/blue_sapphire.htm

Answer 1

hi, wiki says that, it is the inter-valance change of electrons from and between Aluminum ion and Titanium ion and Iron ion which gives sapphire the color.
http://en.wikipedia.org/wiki/Sapphire#Color_change_sapphire

Answer 2

Neat. Presumably there is a way to hash "yellow light is absorbed" on paper. My next question is, what are the equations behind this valence change induced electromagnetic energy absorption?