Coby Madison suggests another important attribute to consider when choosing a diamond is its polish. A diamond's polish indicates how well light passes through the stone. Diamonds with excellent polish are more brilliant than their poorly-polished counterparts. Stones with poor polish possess polish lines that affect a diamond's surface. These polish lines hinder the entrance and reflection of light that either enters or exits the diamond, resulting in a blurred surface.

Diamond polish is graded from Poor to Excellent. Stones that have excellent or flawless polish (EX or E) have no presence of polish lines under 10-times magnification. Diamonds with very good polish (VG or VGD) possess polish lines that are extremely difficult to locate under 10-times magnification. Stones that have good polish (GD, GO, or G) have polish lines that are difficult to view under 10-times magnification. Diamonds that have noticeable polish lines under 10-times magnification earn a fair polish grade (F, FR, or FA). Gemstones or diamonds earning diamond polish grades below these ratings have polish lines that are already visible to the naked eye.