Where can aluminum oxide be found?
Aluminum oxide is corundum. If it has a tiny amount of Cr in it, we call it ruby. Other gems of corundum are sapphire. It is not a particularly common mineral as it needs very low silica environments (otherwise it reacts with silica to form mullite or one of the Al2SiO5 polymorphs). Corundum is found in metamorphic rocks of low silica-content, around low-Si igneous intrusions, and in some igneous rocks called pegmatites. Corundum for sand-paper etc. is made from bauxite which is the main Al ore and is a mixture of Al hydroxides and oxy-hydroxides. These are formed by extremely intense deep weathering of rocks in tropical regions. Check corundum and bauxite on the internet for more details
Sapphire refers to gem varieties of the mineral corundum, an aluminium oxide (α-Al2O3), when it is a color other than red, in which case the gem would instead be a ruby. Trace amounts of other elements such as iron, titanium, or chromium can give corundum blue, yellow, pink, purple, orange, or greenish color.