I accepted the quest to get Herespias crystal ball. But I cant convince her to give it to me. Tried to kill her and then take it - but I cant pick up the ball - the game doesnt let me.
And I wondered why I cant just take the crystal ball at the wizards chamber above the serpent library

can anyone help me with this quest?

Sounds like you had the same problem I did. You will actually have to type in "Orb of clarity". She will then say she will give it to you if you give her the serpent wand. It is not the crystal ball object that is in the window. It looks like an Amulet of Life in your inventory screen.

yes its the orb of clarity not the crystal ball you want. When you ask her about the orb she tells you she will only give it to you in exchange for the serpant wand from the temple.
I to have read that if you kill her she will drop the orb so I saved and then killed her to test and there was no orb so if you have killed her I think you are stumped on this one. You could try taking the wand back to the house and see if the orb appears then as several quest items do not appear until the quest is given, (ring of saints does not appear nect to table leg until you have the quest to retreive it), or the conditions of the reward are fullfilled