Rubies are the hard currency of Mr Football, sometimes earned from completing in-game tasks but mostly available only through paying real money.
A small supply of rubies can be accumulated through time-based releases in Drills and as rewards in events.
Card packs are the main way you spend rubies to improve your list, starting with the National Pack but also variations on that pack for themes, events and special occasions. You can also pay for top-ups of the soft currencies – tickets, credits and coins (but not fame) – and speed up timers using rubies.