In this article talk is understood to be a resource for mathematical learning in school. As a resource it needs to be both seen (be visible) to be used and seen through (be invisible) to provide access to mathematical learning. Lave and Wenger’s (1991) concept of transparency captures this dual function of talk as a learning resource in the practice of school mathematics. I argue that the dual functions, visibility and invisibility, of talk in mathematics classrooms create dilemmas for teachers. An analytic narrative vignette drawn from a secondary mathematics classroom in South Africa illustrates the dilemma of transparency that mathematics teachers can face, particularly if they are teaching multilingual classes.