When you told her 'if she's good she can come down' what kind of 'good' did you mean, presuming she is up there by herself?
Did you mean not screaming and shouting?
Was she?
Did you mean not messing up her room?
Has she?
Just seems a bit of a non-specific statement, tis all...
She might well have meant to aim the stone at you, but she might not have meant to hurt you (ie she was thinking about the journey, rather than the destination).
She may have thrown it because she was already angry and cross about what had happened at school with her friends - she seems to have got herself into a situation she doesn't know how to resolve there.
I sympathise. My DD is also 8. She is largely lovely but does get into vile tempers sometimes. She calls them her 'grumpy moods' and can be really destructive and rude. She doesn't enjoy them anymore than I do.
I try not to escalate it by usually leaving her to it - sometimes by putting her in her room, but just as a short 'time out' to give us both some breathing space.
Then we have a chat about it.
I would say 3 and a half hours is quite a long time - it kind of means no real family 'down-time' this evening before it's bed and then back to school tomorrow and having to deal with her tricky friend situation.
Gah 8 year olds - a real muddly in-betweeny age, dontcha think?