Symbolism, Imagery, Allegory
We wish we could have had a heartstone that could have helped us when we were trying to choose between teenage crushes. How does this thing work? "The closer you get to your heart's desire, the hotter the stone burns" (3.211). In other words, the heartstone tells you what you really want, even if you don't know it yourself.
Come to think of it, that actually could end up being embarrassing, depending on the context.
Anyway, the heartstone in this novel, which was originally owned by Saba's mom, is given to our heroine by Mercy. That alone is pretty symbolic: Saba constantly struggles to make peace with Ma's death. She even blames Emmi for it. It might not be much, but the heartstone provides her with a small connection to the most important woman in her life. And that's not even mentioning its magical powers.
Speaking of those powers, Saba expects them to come in handy when searching for Lugh. The reality is a bit more complicated: to Saba's utter shock, the heartstone starts warming up whenever she's near a charming thief named Jack—she even uses the gem to locate him in a burning building. This is a reflection of how Saba is growing a set of individual needs and desires now that she's been separated from her twin brother Lugh.