When rough cutting facets with 600 mesh I use a 3.5x Donnigan Optivisor. Depending on gem size (and material), leave the meets 0.5 to 2 mm gap.
When pre-polishing I add a 21x triplett loop with the Optivisor. Bring in the meets to 0.02 to 0.10 mm.
When polishing I use a Celestron digital microscope at 80x to 150x. Bring the meets in to within 10 to 20 µm.
I finish the meets to touching (approximately), by continuing with the polish,