- Soul gem
- Greif seed
Kyubey needs Soul gem so that he can collect energy
Only Greif seed can be converted into energy however, after a fight they are completely empty.
Soul gem’s refill the trapped soul with their own grief in order for the cycle to continue.
This makes the evil witch madoka completely pointless because Kyubey would never be able to harvest her power.
If Kyubey can give Homura the power to surpass entropy why is the universe in so much trouble. Why did Homura have a dirty soul gem when she is a already a witch? Again, if Kyubey can create endless power that easily why is the universe dyeing?
Homura is like madoka, because its endless there is no way of collecting it.
In order for Homura to have endless power she would need to be able to release it like a witch.
*if she went back in time as a normal magical girl her soul gem would be black.*
*This theory is backed up by the ending of the TV series*