The article offers a historical ethnographic analysis of a rumor of a resource in the Kitui district of Kenya that was heard during fieldwork on water development practices and technologies. This rumor dealt with a substance known as red mercury, an elusive material that was said to appear as a resource at particular historical and geographical locations. I unravel the details and complexities of this rumor in order to demonstrate how it signifies a particular reflection on the sociality of resource mobilization in the development encounter. This article elaborates how theories of materiality can help to understand the historical embeddedness and particular material qualities of red mercury. It uses the rumor of resources to differentiate the complexities of value and materiality in resources and commodities.