Pearls that were originally strung on silk cord need to be restrung every two to five years, depending on the frequency of wear. Silk cord not only stretches out over time, but exposure to perfume, hairspray, body oils and water can fray and damage the silk cord. When restringing and knotting pearls with silk cord, be sure not to fray the delicate cord or you'll have to start over with a new piece of cord. A needle is the most important tool in determining what size silk cord to use.
Choose from silk cords based on the lower-end of the thickness chart. Thinner cords knot better than thicker-gauged cord when stringing pearls.
Determine the weight of the pearls you are knotting before buying cord. The heavier the pearls, the stronger cord you'll need to ensure the cord doesn't break.
Gauge the size of the hole in the pearl with a needle test. The largest needle that fits through the pearl without sticking is the size to go by when searching for silk cord, as the hole in the pearl needs to be slightly larger than the silk cord.
Avoid knotting pearls with frayed edges through silk cord, as the edges can snag the cord and deteriorate it.