The confrontation between Saruman and Gandalf - "A white page can be overwritten, white cloth can be dyed, and a white light can be broken."
"He who breaks a thing to understand a thing has left the path of wisdom."
This will always be very, very significant to me - the conflict between "the world of men": scientific progress at the expense of the natural world - and the rest of the universe (implied), being either a pastoral existance (I think that's what Tolkien wanted, the tree-hugger) or simply the "living with the world" style of the Elves (not Dwarves, destructive little artificial beings).
The core of Tolkien's definition of evil is "destructive technophilia disguised as progress". The Ring is the ultimate expression of "the will to control" - that's why I love Galadriel's transformation in FotR. It's explicit in "Letters" that it was a test of her resolve - can she pass up the ultimate power to transform Middle-earth into her vision of Aman, via the Ring, and accept the alternative: to lose her "little Aman", Lothlorien, which she's artificially kept "timeless and golden" with her Ring, and diminish back to her basic self, accept her identity, and reman Galadriel? If she passes up the Ring, she absolves herself of the stain of the Doom of the Noldor, and can - finally - see the Undying Lands with her own living eyes again, for the first time in literally unnumbered years.
Watching Lorien makes my heart ache. It's the ultimate expression of yearning, of the ticking awareness of the Long Defeat, of time's passage resisted through main force of will. Glorious, alien...cold, unliving, always looking back, never looking forward. Galadriel was a being of ultimate regret, having finally arrived at maturity and wisdom after a youth of (relative) impulsiveness.
Here I go again...