So I understand that the stones in the story "The Lottery" by Shirley Jackson, are a metaphor but I'm having trouble figuring out what the metaphor is. Can anyone think of a metaphor for the stones?
I need to disagree with the basic premise of your question because I believe you are confusing metaphors and symbols. Let's talk about both, so you are able to use literary terms with precision and apply them to any analysis you do in literature.
A metaphor, simply put, is something you say or write that describes or characterizes something as something else. For example, I might say "His heart was a stone." Anyone reading that knows perfectly well that no one has a heart that is a literal stone. What I am conveying is that this person's heart has some stone-like qualities. This is a person we might also call "hardhearted." I am trying to show a person who is unloving, someone who is unable to engage in ordinary human emotions. Our speech and writing are filled with such metaphors, when we say things like "It's raining cats and dogs," or "You are my sunshine." In "The Lottery," those stones are not being used as a metaphor at all. They are actual stones in the story, in the pockets of the boys, piled up in a corner, and finally used to kill Tessie.
The stones are being used as a symbol in the story. A symbol is meant to represent something else, meant to remind us of something, inside or outside the story. In The Great Gatsby, for instance, the light that Gatsby gazes longingly at is a symbol of his yearning for Daisy. In Huckleberry Finn, the Mississippi River is a symbol of freedom and of journey. In "The Lottery," what are the stones a symbol of? They represent violence, certainly, and they also evoke ancient times, when people actually were stoned to death. In fact, there are still places in the world where people are stoned to death. They represent a lack of mercy, I would say, as well, since this is a dreadful way to carry out a death sentence.
If Jackson had chosen to say the people in the village had hearts of stone, that would be a metaphor. But she uses them to symbolize violence, a lack of mercy, and an ancient and ugly ritual. This makes them symbols in the story. As a literary analyst, it is important for you to understand this difference.