What color are your eyes? No matter how you answer, chances are really good you can answer that question in one word. This is because most of us have eyes that are the same color—but not Abuelita, though. Instead she has one gray eye and one brown eye. We have no clue whether she was born this way or her gray eye is something that developed over the course of her lifetime, but one thing we're confident about is that the gray eye matters.
This gray eye of Abuelita's is a sort of synecdoche; it's a part that stands in for the whole. The gray eye does what Abuelita does, but in a more focused way, letting us zoom in on her as a character.
For example, the first time we see the eye is when Abuelita defends herself against the narrator's doubts: "her pasty gray eye beaming at me and burning holes in my suspicions" (3). The eye has the ability to see the narrator, which most of her family doesn't seem to be able or interested in doing, and it also holds the truth ("burning holes in my suspicions"). Abuelita sees the narrator for who she is, unlike her other family members, and because of this she is trustworthy. The eye makes this all quite clear, doing on a small and specific scale what Abuelita does in general.
That trustworthiness spreads into full-blown goodness when the narrator describes gardening with her grandmother. Here comes the eye again: "[…] I always felt her gray eye on me. It made me feel, in a strange sort of way, safe and guarded and not alone. Like God was supposed to make you feel" (4). This all-seeing eye (like an omniscient god) isn't just a spy; it is a caring, protective organ—a part of a caring and protective person.
Importantly, though, the eye isn't just about who Abuelita is for other people—it's also reflective of her life force. When the doctor checks on Abuelita he looks "into her gray eye, then into her brown one" before declaring that "it was just a matter of days" (6) before she will die. Considered together, Abuelita's eyes are like a meter; the doctor can tell how much of her life has been used up and how much is left just by peering into her peepers.
On her deathbed, Abuelita's eye holds her whole history, everything the narrator would probably like to know:
"Up close, you could see her gray eye beaming out the window, staring hard as if to remember everything." (6)
Here's where the synecdoche really kicks in. The eye can't remember anything—it's just an eye—so when it tries to "remember everything," we understand that Abuelita is taking one last metaphoric look at her life.