For electric guitar, there are also nickel strings that have a warmer tone. Pure nickel will have the warmest tone, but even the 8% nickel wound string will be less bright than the steel strings.

There are also a variety of ways for strings to be wound/wrapped. The way the wrap string is wound around the core string also affects tone:

The profile of the wrap wire also effects tone and feel with: Round wound being the brightest and roughest. Half round being warmer and smoother. Flat wound being the most mellow and smoothest.

On the other hand, acoustic guitars use bronze strings, which are composed of %80 copper, %20 zinc, but there are phosphor bronze strings which are %92 copper, %8 zinc which give a warmer sound.

I have often seen rubber strings on a ukulele, particularly bass ukuleles, and even seen a bass that uses rubber strings.

You can go to this website here for more information on string types: http://www.stringsandbeyond.com/what-strings.html

For electric guitar, there are also nickel strings that have a warmer tone. Pure nickel will have the warmest tone, but even the 8% nickel wound string will be less bright than the steel strings.

There are also a variety of ways for strings to be wound/wrapped. The way the wrap string is wound around the core string also affects tone:

The profile of the wrap wire also effects tone and feel with: Round wound being the brightest and roughest. Half round being warmer and smoother. Flat wound being the most mellow and smoothest.

On the other hand, acoustic guitars use either brass wound strings (often incorrectly labeled as "80/20 bronze"), which are composed of %80 copper, %20 zinc, or phosphor bronze wound strings which are %92 copper, %8 tin which give a warmer sound.

I have often seen rubber strings on a ukulele, particularly bass ukuleles, and even seen a bass that uses rubber strings.

You can go to this website here for more information on string types: http://www.stringsandbeyond.com/what-strings.html