Original sentence: There have been a variety of formulations over the years and the quality of artificial stones continues to increase
Edited sentence: Sword polishing is still most commonly performed with extremely costly and rare natural polishing stones, as the visual results of polising on a synthetic stone tend to produce a mirror-like surface, rather than the grain revealing polish of the natural stones.There have been a variety of formulations over the years and the quality of artificial stones continues to improve, so much so that many carpenters have converted to synthetic stones altogether