The Ruby is a rare ore that you can get from Stone Tali and Rare Ore Deposits.
Recommended Method for Farming Rubies
You are likely to get at least one Ruby when you defeat an Igneo Talus. You have a 50% chance of getting a Ruby when you defeat Battle Tali and regular Stone Tali.
Rubies also have a small chance of coming from Rare Ore Deposits.
You can also purchase Rubies for 220 rupees each from Bladon at the YunoboCo HQ Mineral Market, next to the road that goes south from Goron City. To restock his store, go some place far away and sleep or sit by a fire until morning.