How do you assign credit to "killed" here? By definition, it can't be single-blocked, and if it's a 2HG game and each opponent throws in a 3/4 of their own to block it, which one dealt the killing blow? (In fact, the true culprit is rule 704.5g, which obviously doesn't have a controller.)

There are even other, more complicated ways to cause its death: suppose you have two of these out, and your opponent's Leonin Relic-Warder dies during combat to bring back the Leyline of Singularity it was hiding. Or it took 6 damage, but that's okay because Gaea's Anthem was boosting it up to 7/7...and then something destroys the Gaea's Anthem later on. You also don't have any provisions for a culprit that's neither a spell nor a permanent, such as a cycled Resounding Thunder.

Well then, I guess I'll have to think of something else.

If it's a 6CMC creature, the mana rock it gives you frequently won't matter much as it's at the high end of the curve. Chasmfiends are prized creatures, with everyone out to get them. Have you thought about making it undercosted but the gemheart as a drawback? Can also fill the "undercosted demon" slot in the set.

Juvenile Chasmfiend
Creature - Beast [r]
Menace
When ~ dies, put an artifact token named Gemheart under an opponent's control. It has ": Add one mana of any color to your mana pool."
5/5

Sure, but it has to be green, so it can fit with our beast archtype deck.