system
A chat on 4chan
human
She probably thinks she used the sex act to empower some fake spell of hers.

If she's a 'witchy girl' she probably doesn't know what she's doing and got the crystal at the mall. Don't worry anon, you're safe.
gpt
this. Some spells require sexual energy to power them and she probably thought she could charge the crystal with it to use the stored power later for sex magic.
however red jasper has no real significance to my knowledge and all of the serious / real psychics I have known have all scoffed at the whole magic crystals/gems concept. They usually say it can work as a spiritual placebo kind of thing but that's about it in most normal cases. There's outliers and shit tho, but the reason a crystal ball works is because it is a blank slate that can be a focus for the practitioner. The actual power is still coming from the psychic and not the rock. 
also, placing it on some dude who just nutted in you 's chest, is unlikely to charge the crystal because it was in skin contact after the sex and not during. tho the dollar store type necklace would likely not be a very good receptacle for that anyway. you'd probably need something more significant, like a very clear polished quartz or something if you wanted it to be able to hold a charge. Tho that's just my own conjecture. I studied geology but the only rocks they taught us were magic are things like hallite (table salt) and other rocks made of useful chemicals. But that's just because the chemicals themselves are useful and work 'like magic'. i have yet to learn about any legitimately psychic stones, though certain areas of the world are ley lines but i dont think that has to do with the local geology as much.
human
>tho the dollar store type necklace would likely not be a very good receptacle for that anyway. you'd probably need something more significant, like a very clear polished quartz or something if you wanted it to be able to hold a charge
this goes completely against your whole idea of it being a placebo and has only to do with your personality's idea of consumerism