Underwater mineral dressing units, systems and methods are provided. Material is milled in a water-immiscible liquid which is heavier than water and lighter than the product material. Gangue material is removed by the milling and floats on the water-immiscible liquid to be removed without further processing. Product containing material sinks and is further milled to remove additional gangue material, until the required level of extraction is achieved. Cascades of milling rolls in the water-immiscible liquid allow ever improving extraction of the product material. The rolls are arranged to regulate material flow through the system to remove floating gangue while directing sinking product material to further processing.