My daughter uses them all the time and they always stretch and break. When they break there is a white elastic in them. I guess we have been lucky because those from China are real nasty. I will be on the look out though and hopefully never come across one.
I looked at the insides of a few different ones, and they all had white or black elastic in them, kind of like a square rubber band. They were pretty hard to unravel, probably higher quality than the ones sold in China anyway.