Build the item shop and buy it from there? aha
Modifying my own post to answer my question myself. Gonna use keywords here for anyone searching in the future.
Where to find diamond in survival?
I finally found Diamond at the level of Tuff, 3 blocks or so above serpentine. I found Ruby in serpentine.
I had to strip mine, mine in a straight line, while staring at the mini-map to catch it. Make sure to have mini-map on and a general idea of what the block texture looks like.