Because crystals are not always pure and the inclusions of trace elements is what causes the color, it would seem natural to include a reference to the trace elements, however, for expedience, corundum is corundun whether it is ruby or sapphire and is not essential to the chemical formula. Your milk is just milk, or is it? There are trace elements in milk, we don't list them. Same here. Sapphire is Al2O3 and iron and Titanium causes the color but to list all the trace elements in sapphire would be tedious and different for every sapphire. I hope that helps.