Coral circulated through the everyday lives of antebellum women and men (free and enslaved) as beads, necklaces, bracelets, reef specimens, and other objects. And antebellum coral was almost always more than merely ornamental. Coral has captivated people, across many cultures and centuries, because of its unique physical nature that combines animal and plant, living and nonliving matter, and that grows continually and unpredictably from the interaction of many small and sometimes invisible forces. Coral’s fluid, composite, indeterminate nature and growth set it apart from other natural materials, and it has historically raised new questions, prospects, and possibilities about the natural world as a whole. Yet the many prospects coral raised across centuries and cultures, I argue, share a central feature that enthralled antebellum Americans: coral provoked the recognition that there are limits to our capacity to know, describe, predict, classify, and rank nature, both human and nonhuman. “Antebellum Coral” assembles several written and visual reflections on coral beads and reveals how the cultural and natural history of these small everyday objects challenged the taxonomic thinking that increasingly structured antebellum life. Charting some of the challenges to knowledge, description, and classification raised by the polysemy of coral, I show that coral ultimately gave antebellum Americans new ways to imagine and evaluate an unpredictably fluid material world.