You can get rubies:
●       as a reward for completing all 20 levels in a one-week event
●       as a reward for completing 30, 50, 60, and 70 levels in a two-week event
●       as a potential reward when opening a Gold Chest (the chest does not always contain them)
●       for attaining the third reward in your personal goals in the Airship Race tournament
●       for attaining the third reward in your personal goals in the Pioneers Club tournament
●       for finishing in first place in a League (Silver or higher)
●       in the Clocksville Express event upon reaching the fifth station
●       in a daily gift (the gift does not always contain them)
●       by making an in-game purchase with real money from the Bank, Safe, and special offers.