Cassiterite with Quartz – National Museum of Natural History (NMNH 127924-00). Cassiterite is a tin oxide mineral, SnO2. Cassiterite was the chief tin ore throughout ancient history and remains the most important source of tin today. Most sources of cassiterite today are found in alluvial or placer deposits containing the weathering-resistant grains. Fighting over cassiterite deposits is a major cause of the conflict waged in eastern parts of the Democratic Republic of the Congo. This has led to cassiterite being considered a conflict mineral.