Got it! Attention all: I was wrong about the second part of this.
But the good news is, I´ve found the key! (Yes, like Loki with the
Æsir, I get you into trouble and get you out of it...) The Rosette
Stone turns out to be a descrition of Queen Kormlöð in ch. 154 of
Njáls saga.

Hon var allra kvenna fegrst ok bezt at sér orðin um þat allt er
henni var ósjálfrátt en þat er mál manna at henni hafi allt verit
illa gefit þat er henni var sjálfrátt.

Hermann Pálsson and Magnús Magnússon translate:

"she was endowed with great beauty and all those attributes which
were outside her own control, but it is said that in all the
characteristics for which she herself was responsible, she was
utterly wicked."

This sounds slightly odd as if there´s something missing to tell us
how she was with regard to all the attributes outside her control,
but Dasent obliges:

"she was the fairest of all women, and best gifted in everything
that was not in her own power, but it was the talk of men that she
did all things ill over which she had any power."

Thanks for locating those other sagas, Patricia. If you want to
have a go at tracking down the exact quote, type "illa gefið" into
Google and check the first couple of hits. I don´t know if your
translations have the same chapter numbering as these online texts
(if not, look out for a bit where Halli goes to claim compensation
for his brother Einarr from a certain ill-given jarl, also called
Einarr). By the way, is your Sarcastic Halli taken from they
Flateyjarbók version? There´s another version in Morkinskinna, you
see, which I think doesn´t have the phrase we´re interested in.

LN