Sea glass consists of discarded pieces of broken glass that have become weathered by time and tide and formed into smooth, often colorful gems that wash up on beaches. Many people walk along beaches carefully looking at the ground, as they go to see if they can find some sea glass.
This act of looking for sea glass becomes one of two very important metaphors in Anita Shreve's novel. As the book's protagonist, Honora, says, the problem with looking for sea glass is that you never look up, you never see the view, and you never develop a broad perspective. In metaphorical terms, this applies to life itself. If we only ever concentrate on what's in front of us, then we'll miss out on so much of what life has to offer.
Sea glass is also a metaphor for the indestructible nature of the human spirit. Just as the glass that makes up sea glass cannot be broken by the power of the sea, so Honora's spirit cannot be broken by the many adversities that she endures.
Despite everything that life throws at her, Honora lives to fight another day. The unborn child kicking inside her offers hope for a better future, as does her leaving for Boston. Honora is like sea glass: colorful, beautiful, and unbreakable.