You don’s say what mesh diamond you have tried. You would need
50,000 or finer for a good polish. I have doubts about the efficacy
of “tin plate” although it might work if the tin layer is thick
I have had no problem getting a good polish on spinel using aluminum
oxide on a solid tin lap, also with 100,000 mesh diamond on BATT or
Darkside laps, using light pressure to avoid facet rounding. The
latter laps are available from many dealers in lapidary equipment, or
from Jonathan Rolfe, the manufacturer. Many other combinations will
work, too, but I have not tried them myself.
I hope this helps.