She is described using the following similes:
- Her face is said to be as bright as snow.
- Her eyes are "miraculous bits of violet amber" -- they have captured him like amber captures bugs
- Her face is like the dial of a small clock that knows exactly what time it is -- you can see it even in the dark.
I think the first and the third of these are meant to portray her as a creature of light (as opposed to darkness). She is bringing light and understanding into Montag's life. That is where the second simile comes in -- she has now captured Montag's attention and is going to change his life forever. Finally, part of the third simile is the certainty of the clock. Clarisse is also very certain that she is right.
So between them, the similes are meant to show us important aspects of Clarisse's personality and role in the book.