Done, but not trivial - the machinery is well beyond DIY efforts... many examples
http://lmgtfy.com/?q=optical+mirror+machined+aluminium
See for example
https://www.spiedigitallibrary.org/c...200.full?SSO=1
What makes glass special is that its so stiff, yet easily abraded with abrasives to produce accurate spherical surfaces, in a process that is incredibly simple.