The sand-covered wooden surface was wetted with water, and the gemstone was rubbed against it. In this case, the degree of polish was limited by the lapidary's ability to sort out the larger grains of silica. Eventually, cutters found another polishing agent called emery. Emery is an impure granular mixture of corundum, magnetite, hematite, and spinel. Corundum, consisting of the gem minerals sapphire and ruby, is very hard and will cut or polish most minerals below it on the hardness scale.
Spinel is ranked just below corundum in hardness while magnetite and hematite are much softer.
Later still, diamond was found in India and became the polishing agent of choice when working with the harder gemstones. Diamond particles could even be used to cut and polish the surfaces of gem-size diamonds. The procedures for grading grit sizes improved slowly over the ages, but all information having to do with gem cutting and polishing was secretly held by lapidary groups called guilds. Polishing, Newton and company insisted, was an abrasive process. In polishing, they said, the gemstone was rubbed against particles of polishing agent and scratches resulted.