The phrase "so little is a stone" in line 9 of the poem "Burning the Old Year" by Naomi Shihab Nye is an example of metaphor.
Metaphor is a type of figurative language in which a word or phrase is applied to an object or action to which it is not literally applicable. In this line, the word "stone" is being used as a metaphor to represent something small and insignificant. This line is comparing the small size of the stone to the significance of the old year being burned away, suggesting that even something as small as a stone can have a big impact or meaning.