Pretty much the first thing that will strike any attentive reader about this novel is that the author is doing something odd with the narrative voice. It seems to be Daisy Goodwill Flett narrating her life, but then again how could it be, since she is narrating things she could not have known? And the voice jumps back and forth from "I" to "she" in a startling manner.
If you want an academic discursion on the subject, go (for instance) here: https://journals.lib.unb.ca/index.php/scl/article/view/8225/9282. I'm no longer given to essays in that vein. But I'll just note that this same phenomenon, which I would have taken as startling lack of skill (and editing) in a fanfic author, is the beginning of a really delightful excursion through an autobiography that's all about the emptiness and impossibility of having enough of a grasp on your own story to write an autobiography. What keeps that from being either dry or dismal is a wonderful set of characters, and a strongly-managed set of central metaphors clustering around, you guessed it, stone - but also around flowers (her name, after all, is Daisy, and her great talent in life is as a newspaper columnist about botany).
It was pure serendipity that I have been reading Lauren Bacall's autobiography at the same time as this novel, and I was struck by how often Bacall's musings on her life and her aging intersected with the same themes being explored in Daisy's fictional and metafictional story (even though the "I" disappears fairly early on in the proceedings, and we are left, a bit more comfortably, with a 3rd person omniscient narrator we can jog along with).
I'm not quite sure how to express this, but after reading this, as well as other Shields novels, I tend to feel she's kind to her characters. They certainly have their pointy bits, but they are not the objects of satire. And they're always capable of change. Even Magnus Flett, who gets very little to do except be horribly grouchy and a stereotyped nasty husband in the beginning, has a vulnerable spot (his runaway wife Clementine's copy of Jane Eyre, discovered after she's gone) which ends up being his sustenance and his obsession in his very extreme old age. Cuyler Goodwill builds a memorial tower to his dead first wife (Mercy Stone) when he is a romantic young man, and then attempts, unsuccessfully, to repeat the act when he has become a successful businessman later in his life.
Daisy herself is the epitome of the changing personality as she visits the different epochs of her life, and that is quite in line with the theme of the autobiography that doesn't manage to draw the lines of sense and reason around what is, perhaps, a not terrible sensible or reasonable thing, a human being over time. And yet there are strands of understanding and memory that link each time to the others and convince us that it is Daisy at each turn.
I haven't really succeeded in conveying why The Stone Diaries got my rare fourth star, let alone setting out my understanding of what the author is "trying to do" (horrible, condescending phrase). But I did enjoy it very much, and I urge it upon anyone who has ever read an autobiography or a novel in the form of an autobiography, and wondered if, in the end, it had conveyed "the truth", or perhaps, in looking for a single "truth" about a life, it had given us a hollow structure instead.