It's in one of Immortus's dialogues:
http://i204.photobucket.com/albums/bb6/Enteithegreat/Kang%20the%20Conqueror/Noimmortus.jpg
Maybe I should've stressed this in the respect thread.
Yeah, but that's the crystal, no? Not the Heart of Forever.
I'm trying to say, like, Thanos created that synthetic soul gem, but did so by siphoning power from all the other soul gems into a receptacle he created.
What Metron did was just create 'a map of creation...in miniature', at least so far as we know, which is distinct. So far as I know -- and I could be wrong, I'm not nearly that versed in Kang/Immortus stuff -- but the Crystal was created from the Heart of Forever, which is like...like I soul gem, or the heart of the universe, or what have you...a cosmologically created repository of power, that Kang (or Immortus, or whatever) weaponized.
Does that make sense, the distinction I'm making?
Yep, but Kang built the Heart of Forever since the Forever Crystal, as you said, was built by Immortus. Kang even went on explained that it worked thanks to the Time Construct he also built -Chronopolis-. It's a time engine originally built by Kang, that Immortus later stole but the HoF, as Immortus said was studies since Kang built it. It's supposed to be a machine made by Kang.
The Heart of Forever depends on the universe in which it's used, it's a machine, but the way for this machine to work was devised by Kang, following your example it would be as if Kang built the Soul Gem more than something channeling such power.