Use the jewellers saw, with a blade gauge depending on what kind of cut you are doing. Cutting out shapes = thin, cutting lines that need to have definition = thick. When cutting, break often so that the saw blade does not heat up enough to break or bend. Keep the blade tilted backwards, and rub it gently onto the pearl, applying NO PRESSURE. It will cut well if sharp. If cutting squares, always cut slightly bigger to allow for sanding down of sides (with metal sandpaper).
Be careful to use a good mask when cutting pearl — whether or not the dust is carcinogenic, getting bits of it in your lungs isn’t going to do them any good.
TODO: Find citations RE toxicity of MOP