I'm going to assume you mean steel guitar strings. Steel is a mix of mostly iron with some carbon, but there are different amounts of carbon used and those variations do have an effect on the sounds produced. I found an article (sorry, it's not available online) about the acoustics of steel piano strings that stated that the higher the carbon content, the higher the quality of the sound. I have heard of steel guitar strings that contain a significant proportion of chromium as well. Supposedly, these strings are less likely to corrode, which would cause defects in the string and have a negative effect on the sound produced.

The tone of the strings is determined by their length and the amount of tension they are under (not much good plucking a loose guitar string of any length). This is why you have those knobs that tune the guitar: they add or release tension to the strings.

Different compositions of the guitar strings would change the tension they are under and are able to withstand, so that would change the tone. The tone has nothing to do with the material that the thing doing the plucking is made of, however.