This painting is an excellent example of a focal point -- the high contrast of the bright white pearl against shadow, its punctuation mark.
I recently bought a wonderful book that is a collection of poems based on Vermeer's portraits, and has a reproduction of each painting beside its corresponding poem. Here is an excerpt from Girl with a Pearl Earring, by Marilyn Chandler McEntyre, from In Quiet Light, Poems on Vermeer's Women:
See how she turns to greet what comes,
surprised but untroubled, not quite
welcoming. She looks askance
at one who has, unasking, disturbed her
solitude. Her greeting concedes what it must,
but she remains turned to purposes of her own.