From antiquity until the eighteenth century, most of the world’s precious stones were mined in South and Southeast Asia. Europeans had long associated these parts of the world with mineral wealth, but it was not until the establishment of direct maritime connections to Indian Ocean trade in the early modern period that European scholars were able to investigate gemstones in the places they formed. Over the course of the seventeenth century, Dutch and English East India Company networks created new opportunities for jewelers, goldsmiths, and gem merchants to survey, map, sample, and compare mineral deposits across the globe. I show how participants in the gem trade across Eurasia became crucial sources for European scholars because of their knowledge of precious stones and the places they formed. Using gems as a case study, I argue that the global scale and cross-cultural nature of early modern trade enabled new questions and methods for understanding the composition of the earth. The techniques for mapping and exploiting the world's mineral resources first developed through cross-cultural commerce would become central to the geosciences as they were formalized in the late eighteenth and early nineteenth centuries. By focusing on European engagements with South and Southeast Asian environments, economies, and expertise, I demonstrate that the early modern earth sciences cannot be understood, nor their successful application be explained, without the context of expanding global trade and preexisting knowledge of gems in the Indian Ocean world.