Answer:
Like all US nickels except the famous "war nickels" of 1942-45, buffalo nickels are made of an alloy of 75% copper and 25% nickel.
Regardless of urban legends and internet rumors, the US never made any other silver nickels except those struck during WWII. There were in fact tiny silver 5-cent coins called half-dimes but they were discontinued in 1873 and were never called "nickels".