Who needs crockpots and Pottery Barn registries when you've got gold pieces? We learn in the first chapter Mattie's maternal grandfather gave two gold pieces to Mattie's father as wedding present. Together they symbolize marriage, love, union, and social approval. Because Frank Ross carried them "concealed in his clothes" (1.11) during his travels, they symbolize the value he placed on his marriage, his wife, and his family. This little detail helps make him a sympathetic, innocent seeming character, and is just adorable.
When Frank dies, both coins disappear, symbolizing the sudden disappearance of what was a happy, whole family. They don't just disappear, but are presumably stolen by Tom Chaney after he shoots Frank, symbolizing his "stealing" Frank's life. But, one of the coins reappears. Rooster finds it in the pocket of the dead Emmett Quincy (one of Ned Pepper's gang). Now, it's a sign helping to lead Mattie to her Papa's killer. When Rooster finds the coin on Quincy, Mattie knows they are properly on Chaney's trail.
But then look what happens: "I kept [the gold piece] for years, until our house burned" (7.370), Mattie says. If Mattie's journey is a coming-of-age, then this might symbolize the moment when she's able to move past her father's death, or the moment when she truly comes of age. Just check out how casually she talks about it. Maybe writing this story has helped her come to terms with both her father's death and her own journey.
Of course, the second coin is never found. Just like Mattie's poor left arm, it's a casualty of that whole troubled time—and a reminder that true grit requires sacrifice.