Here are some common materials used for different parts of a snap button:
Socket (female part)
Made of brass, nickel silver, or plastic. Brass and nickel silver are most durable.
Stud (male part)
Typically made of nickel silver or brass for strength and corrosion resistance. Plastic studs are also used.
Cap/Bale (top part of stud)
Usually made from the same material as the stud for matching appearance. Nickel silver and brass are popular.
Spring
Small tension spring inside the socket that keeps the stud and socket connected. Steel is commonly used due to its elasticity.
Plunger (pushes stud through socket)
Plastic or metal like brass is typical. Must be durable enough to withstand pushing force.
Back plate (on socket)
Metal such as brass, nickel silver, or steel provides stability. Plastic back plates used for light-duty snaps.
Solder (joins parts)
Tin-lead solder is a typical metal alloy used for sealing and joining mating snap button components.
Finish
Nickel, copper, bronze or black oxide finishes applied to metal parts for corrosion resistance and appearance.