These “rubber” cords are not rubber at all but one of a family of
plastics called elastomers. There are several families of elastomer
(butyl, nitrile, silicone, polyurethane, fluoroelastomer, etc.) and
dozens of formulations. Visually there is no way to tell which is
which but the properties vary greatly with the family and
formulation. The trick is to find out which one(s) are suitable for
use as a necklace cord. I can just about guarantee that the folks
selling this cord did not do any research and just bought black
“rubber” cord at the best price they could get. So it would not
surprise me at all to see different elastomers used in the same
parts sold at different times by the same vendor. Some of these
materials are affected by sunlight and become brittle, some loose
elasticity as the plasticizers in them evaporate, some cannot
tolerate petroleum oils others are sensitive to alcohols and on and
on. Anyway to find a long lasting cord for this purpose you will need
to do some research.
James Binnion
@James_Binnion
James Binnion Metal Arts
360-756-6550