Hey, so this is just my opinion but I don't know a spell or curse like is possiblebut it wouldn't hurt to try and make one though, it's said making your own spell is stronger and powerful I'll link some links that can help you.
Fertility spells can be a wonderful thing... Or a terrible one. You really should think carefully about what you really want from this spell.
Do you actually want her to be unable to have children?
Or do you want her to be unable to have children till she's ready or that having children would be a positive on her life?
(Or even, the one thing no one likes to ask, do you just want to hurt her in a terrible way?)
I will tell you this, even before you consider: Yes amethyst is a healing crystal. It's usually used for divination, projection, and 3rd eye control, but it can heal. I'm not quite sure why it's being used for an anti-fertility spell.
After you consider what you want, it's best to use that knowledge to pick your spells wisely.