>>280521124
It's mostly from other metal ions substituting for a small portion of the aluminum atoms. There are several different ways this can modify the color ( http://www.minsocam.org/MSA/collectors_corner/arc/color.htm ), but for Corundum it's usually either electronic excitation of the impurity atoms or charge transfer.
>>280522486
It's normally all Al, the blue is from iron and titanium