Thanks for the replies, guys!
The string is actually quite simple. It's basically 2 identical renderable helixes (shape) rotated so the sit inside of each other. The knots are just bit more complicated: I drew the know using a curved spline and with the path follow modifier I conformed the straight string to the shape of the knot.
The material is ridiculously simple, I scanned a real 10 cm string and used it for the diffuse. Nothing else. ;-)
BTW, I also used Hair&Fur for the tiny hairs along the string. Nothing fancy.
Cheers!