If you're intrigued by band gaps... get some cheap synthetic Ruby (you can get a 1.5 carets for $40-50) and see what happens when it gets hot/cold. Ruby is ju

If you're intrigued by band gaps... get some cheap synthetic Ruby (you can get a 1.5 carets for $40-50) and see what happens when it gets hot/cold. Ruby is just Sapphire (Aluminum Oxide) with Iron impurities that make it red. It is red because blue light (everything above the band gap) is absorbed! When you cool the ruby (e.g. in liquid nitrogen) it will turn pink/orange as it shrinks and the absorption band moves to shorter wavelengths. When you heat it (e.g. with a propane torch) it will turn deep red to black as the crystal structure expands and the absorption band moves up in wave length from red into the IR. Remove the heat/cold and watch it slowly come back to the ruby color you expect!