Strongest thread for stringing with knots?

I have some smallish rose quarts beads I want to string with knots in between, but I'm having trouble finding something strong enough. They belonged to my grandma, so I'm paranoid about it breaking and losing beads. Does anyone have suggestions for strong, stretch resistant material that looks nice tied in knots? Pic of beads attached; I used some nylon embroidery yarn but I don't think it's very strong, and it's stretched a lot