Rubies are a premium currency in Battlesmiths: Blade and Forge.
 
You can earn rubies by doing either of the following:

Currently, rubies can only be used to purchase some customization items for your Heroes, such as capes, breastplate and helmet decor etc. Some of the items are available at https://battlesmiths.com/, while others are available in the game. If you want to use them, go to customization in the Barracks and click on the pencil icon next to head and breast armor slots.