In The Hobbit...

[doug*platypus]

While far from infallible, it seems that in this case the Companion has hit on a very good point. The fact that Trolls seemingly returned to stone seems to hint at the fact that they were created from that stuff, rather than being corrupted Ents (although I'm not convinced that Treebeard thinks Trolls were made from Ents).

They were creatures of the Darkness that ruled Middle-Earth when Melkor was unchecked and only Valinor had the light of the Two Trees. His spells were either not powerful enough, or did not take account at all of the Sun. Arien had a strong power against even Melkor, after all. Maybe in a way, her 'magic' was stronger than the spells that kept Trolls animated, in much the same way as Merry's barrowblade destroyed the spells that let the Witch-king survive in physical form.