Plum purple is a difficult color for the glass used in seedbeads, which typically is the more dull or brownish purple from use of manganese as a colorant.

"Sol gel" coatings seem to be the norm now for purple Czech seed beads - you can see them at the Preciosa site. Very appealing, but will fade in sunlight. Watch out for anything labeled "Terra," it's dyed.

There is also a sputtered (?) metallic coating ("Crystal Colored Luster") that produces a violet purple over colorless transparent glass, which would of course be abraded off in a tumbler. Loupe inspection reveals this coating.

https://preciosa-ornela.com/images/pdf/Rocailles-Sample-Cards.pdf