Yes, string is a common noun, a Word for any string of any kind.
A proper noun is the name of a specific person, place, thing, or a title; for example:
- David Gage String Instruments, New York, NY
- String Bridge, Exeter, NH
- Boston Spring Players (orchestra, non-profit), Watertown, MA
- "A Cat, A Hat, and A Piece of String", a book of short stories by Joanne Harris
- "Man on a String", 1960 movie with Ernest Borgnine
