Diamonds

I just find strip mining at around y13 is good, be sure to get a fortune 3 pickaxe, and if you haven't already apply for our $5,000 builder bonus as a helping hand to afford one.

https://ecocitycraft.com/forum/threads/ecocitycrafts-start-up-bonus-free-5k-new-returning-players-only.184842

Also, take a look here for some guides on efficeint ways to mine.
https://minecraft.gamepedia.com/Tutorials/Mining

 

Thought it was y11

 

I prefer y13 as lava spawns below there.

 

Y=13 i find is the best

 

I have the best luck at y11

 

my dad finds a heep of stuff at Y8 lol like 5x more that Y11