'It stops existing.'

"Does a soul gem, when a magical girl runs out of magic, dissolve?"

'No.'

"Then what does it do instead?"

There's a long pause. 'Most girls don't like knowing this.'

"I wanna know. About soul gems and why Cecilia was overlapping the witch and near the grief seed - "

'Magical girls who run out of magic die, and stop being magical girls, and their soul gem stops being a soul gem. Witches come from magical girls who die with their soul gems intact.'

"The grief seed - it's Cecilia?"

'Witches are not magical girls. Grief seeds are not witches.'

"That's not how humans do being people!" She clenches her fists, shaking - 

But she's not despairing.

She's mad, and she feels almost more hopeful than she did when she made her wish, full to the brim with I will change this - 

'I'm sorry. That's how we consider people.'

"I'm going to change it! I won't - I won't let Cecilia be destroyed, and I'll fix her someday!"

'The grief seed will hatch again, even if you don't use it.'

"Then I'll fight her again! And again and again, as much as I have to!"

'That's your choice,' Kyubey acknowledges. 'You will need to be careful about where and when she hatches. It's risky.'

"I'll take on all the risks I have to."

'You should still hunt,' he says. 'Witches are a danger to humans, and you won't do well with just one grief seed.'

"Do all witches come from magical girls?"

'No. Most come from familiars.'

"And where do familiars come from?"

'Witches make them as they grow.'

"How do witches grow?"

'Witches feed on despair. They get bigger if there's a lot in an area, or when they put a Witch's Kiss on someone, especially if they manage to kill someone."

There's an odd thought that echoes back to her with that... But it flees when she tries to poke it. 

"Why don't most magical girls cooperate?"

'Almost all witches can be taken down by a single magical girl. Grief seeds are very valuable, but if you only have one, you have to decide who keeps it, and if you have two, you have to decide who gets the bigger one. And if two magical girls are hunting the same witch, they might not want to make that negotiation.'