Every facet's surface must be perfectly smooth for light to pass through and reflect without interference. A diamond with 'Excellent' polish has no visible scratches, burn marks, or polish lines on the surface at 10x magnification. If the surface is scratched or dull, a microscopic 'noise' forms that blocks light, reduces transparency, and weakens brilliance.

GIA grades polish in five levels:

• Excellent: No surface defects visible under 10x magnification. Light moves freely in and out of the stone.

• Very Good: Defects extremely difficult to see under 10x magnification. No noticeable impact on luster.

• Good: Faintly visible at 10x magnification. Negligible impact on brilliance to the naked eye.

• Fair: Visible defects under 10x magnification that may faintly affect luster.

• Poor: Defects visible to the naked eye that noticeably reduce the diamond's shine.

Experienced gemologists identify specific types of surface defects:

• Burn marks: Occur if the polishing wheel moves too fast and heats the facet. Appear as a cloudy patch.

• Abrasions: Subtle scratches along facet edges. Most common on older jewelry worn daily for many years.

• Lizard skin: An irregular, wavy surface texture – a sign that the diamond was polished in the wrong direction against the crystal structure.

• Polish lines: Fine parallel lines from the polishing process. Almost never visible without 10x magnification and rarely disturb brilliance.