How can polished and frosted facets be created on a gemstone? There are a number of gemstone designs that have some of the facets left unpolished or frosty.

From software I created, the coordinates of each vertex and the equation of every face is known.

This is why I use Boolean Difference to create my gemstones - one facet at a time.

Since I do one facet at a time, is there something that I can do to make the facet frosty like a roughness of 0.2?
If I understand correctly, you want certain faces of the same object frosty while others are glossy.  Is this what you're thinking?  If not, please add additional info to the question:

1.) Select which faces you want frosty:



2.) Assign the selected faces a separate shader with roughness of 0.2: