An irregularity of the granular structure, a knot can give a cutter much trouble in the fashioning process of a diamond. While grinding material away, an encountered knot (also spelled naat) will cause the facet to stop cutting almost entirely. The cutter must then readjust his direction of attack to regain progress in polishing. Sometimes a knot appears out of nowhere, but probably more often, an included crystal has opened to the surface during the grinding process, leaving the tell-tale sign of geometric lines on the surface.
Often a facet that contains a knot is poorly polished, due to the difficulties in finding a good cutting position. Trailing lines will often be detected around the knot. Expect to pay higher cutting fees if a re-cut is necessary on a knotted stone, since this will usually take much more time to complete. For the most part, knots are detected beneath the surface of a stone and therefore are classed as inclusions.