Got my Ruby in Korroloka Tunnel. They drop off of the Thread Leeches there.
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Did you lose faith? Yes, I lost faith in the powers that be. But in doing so I came across the will to disagree. And I gave up. Yes, I gave up, and then I gave in. But I take responsibility for every single sin. ♪ ♫
The drop rate for the Carbuncle Ruby seems to be much better in Bubirimu Peninsula.
The area I found mine was down near where the Bard advanced job quest finishes - to the West of Mhaura. My ruby dropped on the 4th kill believe it or not.