First of all, there is no such thing as "alumina oxide". Alumina is aluminium oxide. Melting alumina is pretty hard, its melting point is above 2000 degrees Celsius. It's so high that people use it as a container (crucible) for melting other stuff. Such high temperatures are not easily achieved without melting a bunch of stuff around it.
Let's assume that you did manage to melt it, maybe using one of those fancy levitation laser techniques. Your mold has to be heat resistant and inert so it will not melt or react with the molten alumina. Graphite is usually a good one.
Now, you were asking about the structure. Assuming that you started with regular crystalline alumina (aka corundum), then yes: the resulting material after cooling in air will be the same corundum. Amorphous alumina (glass) will only form by extremely rapid cooling, for example dropping a very small amount to ice cold water. In all normal circumstances it will be crystalline.