A Zippo lighter flint-wheel ignition lights up when the rough-surfaced wheel is rubbed against  the flint which produces the spark which ignites the gaseous fluid in the wick. The flint is made of ferrocerium, a man-made metal has to be replaced if the Zippo is in use regularly.   In the late 1930's Zippo sold flints in paper envelopes, 2 flints for 5 cents, and later in the early 1950's it introduced the flint dispenser which with customer imprinting became a promotional and marketing vehicle in its own right. 

A helpful chart for the identification the different types and styles of packaging used for Zippo flints made by the Japanese Zippo expert Hiroshi Kito can be found in David Poore's book Zippo: The Great American Lighter , Access #174.