Violin strings are either steel, silver-plated or chrome-plated steel or aluminium wound on nylon or gut. Gut strings are supposed to sound better, but are more expensive and break more easily. If I remember correctly the top G string is a plain steel string anyway. Nylon string sets are mostly used by beginners. But I'm really not sure, it's been more than 6 years that I touched my violin, or even looked at it.
Oops, yeah! It's the top string alright, now I typed G instead of E, sorry. Well, that shows I'm a bass player in my mind now violin tuning is of course G D A E (in 5th).
Most violin players use our strings and the options for the E string (among the T-I's) are plain steel, aluminum wound on steel core, chrome wound on steel core, chrome steel, tin-plated steel, chrome steel with gold plating and carbon steel with tin plating. Gut strings are still used by some but they are subject to climate changes and break easier. I lost a link to a site that shows how gut strings are made. It's a tad unsavory so for the faint of heart perhaps I should stop looking.