Hi James,
Gaza superba is the only species that gets ooohs and ahhs from more
than a foot away, because the rest of the Gaza species are rather
small shells.  Like many small shells though, some of them are quite
beautiful.  Gaza sericata, which you asked about (from Japan) is a
very pretty shell.  It has the pearly lustre of Gaza superba, but has
intricate markings of yellow, tan and brown as well (unlike G. superba
which is unicolored).  However, G. sericata is only about 20 mm in
diameter.
Regards,
Paul M.