I have tried so many types of thread when it comes to creating my own jewellery and designs for clients.The problem is that so many threads fray if beads are heavy or inside the bead is rough. Lots of my customers ask for Silk thread but unless the beads are very light the silk thread in the picture on the left is quite easily frayed.
Softflex extreme is a twisted wire thread coated in nylon.I like to use this for general restringing.
For something very unique there is even a cork thread but it is very thick so no good for beads with small holes.
The nylon twisted threads are the best and can be cut with a special heat cutter allowing very secure knots.It does not stretch but does fray occasionally.I try to double up strands where possible.
C-lon can be bought online from this shop