Ambrelane

I have a problem at the end of Mourning's end part 2, I got to the death altar and put my white cristal on it. However I could not retrieve the cristal after that so I am not able to finish the quest. I tried to go back to the black one and have a new piece but it says I don't need it. I went back to Arianwyn but nothing happend, I went to Eluned but I had no option either. So I am lost ... what can I do ? did I do something wrong ?