When setting colored gemstones, it is extremely important to place the stone in the mounting in exactly the same position each time you test fit the stone while cutting the seat. The reason is due to the fact that most colored stones are cut irregularly. To assist you in this placement, draw a line on the crown of the stone with a fine tip marker. Then line-up this mark with a distinctive design element of the mounting, a similar mark with a felt tip marker on the mounting, or a mark made on the holder holding the mounting (such as a deep file mark filed into your ring clamp). When finished setting, any remaining ink can be removed with alcohol.