Symbolism, Imagery, Allegory
Sure, Uncle is a pretty miserable cook. He's no Anthony Bourdain, not by a long shot, but that doesn't mean that the stone bread isn't an important symbol in the novel. It represents both Uncle and difficulty.
Uncle
On the most basic level, the stone bread is a symbol of Uncle Sam. He's the one who makes it. Not Obasan. Which, if you think about it, is kind of unusual since it doesn't seem that he cooks anything else.
Since it is the only thing he makes, and no one else makes it, it is totally attached to him. So everything about it reflects on his personality. Check out our analysis of Uncle Sam's character for more info on what makes Sam so Sammish.
Dealing with Difficulty
Naomi just thinks that Uncle's bread is hard because he's not good at baking, but there's more to it than that. Stone bread isn't Uncle's invention. Stone bread is actually a food that has been made by people who don't have much to eat. In addition to the regular ingredients, it uses any other leftovers that you could find. Kind of like a pot luck, or soup in the story "Stone Soup". The result is a hard loaf, but it is more nutritious than regular bread. In other words, stone bread is a way to make the best out of what you have.
Things get interesting when stone bread is further associated with Aunt Emily and her parcel. The parcel is as heavy as stone bread. When Aunt Emily give Naomi documents to read, she says:
"Read this, Nomi," she said from time to time, handing me papers as if they were snapshots. […] "Give you something to chew on," she said. She was eating a slice of Uncle's stone bread with a slab of raw onion. You're the one with the strong teeth, I thought to myself. She did have strong teeth. And a tough digestion. (7.40)
Aunt Emily's tough digestion doesn't just apply to the hard bread, but also for dealing with terrible memories from the past. She's one tough cookie. She's not afraid to deal with the difficulty that she has experienced, and she wants to share her feelings with everyone. On the other hand, Naomi refuses to eat the bread. Just like she refuses (at first) to deal with the sorrows in her past.
But by the end of the novel it seems like Naomi is finally able to deal with her memories. So, do you think she'll like the stone bread now? Um, probably not. It still sounds totally nasty to us.