Often mistaken for polishing lines, internal graining lines are easily distinguished by their length and depth. Polishing lines occur on the surface of an individual facet running in different directions from one facet to the next. Graining lines run over the width or length of several facets and lie beneath the surface of the diamond. Commonly found printed under comments in a GIA report, it reads: “graining lines not shown”.
Often called twinning lines, wisps and knot lines. This irregularity in the granular structure of the diamond appears as parallel lines that can be colorless or tinted in off-white or other subtle colors, which in turn, can dramatically affect the clarity of the diamond.