This hero, I assume: https://us.diablo3.com/en-us/profile/Scott-1463/hero/135235150
You aren’t supposed to… but I bet you’re getting a lot of rubies.
Refer to #20 of Things That Are NOT Bugs to learn how Emanate works.
If you want diamonds, you could put a diamond in the Helm of command being worn by the hero… instead of a ruby. Alternatively, you can use the cube recipe to convert 9 gems from one type to another (such as rubies to diamonds).
Not a bug.