The Silmarils burned Carcharoth and Melkor because they were hallowed artifacts. But when Ungoliant sucked the life out of the Two Trees (two of the most hallowed items in Arda, I'd venture) she showed no ill effect at all. In fact, her body and power swelled as she drained the Trees. From this, it would seem that the Silmarils, too, would have simply engorged her even more - perhaps to the point of bursting.
Now that would have been messy. [img]smilies/eek.gif[/img]
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The Barrow-Wight
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