Rubies are the game's hard currency, and, as such, are not easy to win.
You can win Rubies from PvP. You can earn anywhere from 3 to 300 Rubies per week, depending on your league and tournament ranking.
The only other place in the game where it is possible to earn a Ruby is at the Wishing Well, a hidden building.
You can also purchase Rubies at the Shop.