If your pre-polish is good enough, you may be able to see asterism
under a bright point light source if the garnet is wet, and if the
stone is oriented properly. If no star is visible, proceed to a
polish and check again. Garnet asterism is usually weaker than what
one sees in star sapphire.
Yes, the finished cab is still garnet and can be sold or used as
such, but will probably be very dark.