Symbolism, Imagery, Allegory
When John comes home the first time, knowing that the war isn't going well, he orders Louvinia to pack up the silver into a trunk and Joby and Loosh to bury it in the orchard. This is the first signal that John's nervous, and it lets us know that Loosh is right: the Union is closing in on the Confederacy.
When Granny decides to evacuate to Memphis, the trunk is utmost in her mind. The way she guards it, having the slaves dig it up and bring it to her room the night before they leave, reveals her suspicion, not to mention the distance between master and slave:
"I wish you'd tell me why you got to dig hit up tonight."
Granny looked at [Louvinia]. "I had a dream about it last night... I dreamed I was looking out my window and a man walked into the orchard and went to where it is and stood there pointing at it," Granny said. She looked at Louvinia. "A black man." (2.1.13-16)
Louvinia treats Granny with familiarity; there's not much deference or fear in her tone when she questions Granny. So when Granny explains that she must dig up the trunk because she doesn't trust the slaves, it puts a distance between her and Louvinia that might have been there before, but wasn't out in the open like this.
Later, when the trunk is re-buried and the Union soldiers show up to burn down the house, Granny would rather risk her life to save the silver than stay safe and hidden in the barn. It's Louvinia who holds her back. So Granny's after the silver, but Louvinia is more worried about human life.
When Granny finally makes it to the Union camp to try to recover her belongings, she lists the trunk first: "They took the silver and the darkies and the mules," Granny said. "I have come to get them." (3.3.4). The silver, which is a symbol of the family's wealth and their past life, is at the same level of (or slightly higher than) the slaves and mules.
The loss of these objects (and people, even though Granny talks about the slaves like they are objects) is the loss of their status, their past, and their way of life. The trunk of silver is just a substitution for her: since she can't go back to the past, she just tries to get back her silver and the memories it represents for her.