I’ve found that monocrystalline diamond suspension can be useful when a stone is already properly pre-polished, and the goal is to remove the last fine scratches without taking too much material.
For harder gemstones, the particle size and consistency matter quite a bit, but so do the lap and how the suspension is applied. Keeping the lap clean between grit stages is also important, especially when working with very fine diamond.
This monocrystalline diamond suspension reference covers different diamond suspension options and their use in lapping and polishing.
What grit do you normally use for the final diamond stage, and which lap has given you the most consistent finish?