Refractory minerals — including kyanite, sillimanite, andalusite, and mullite — are aluminosilicate materials that withstand extremely high temperatures without melting or deforming. They are essential for lining blast furnaces, electric arc furnaces, glass tanks, and cement kilns. Because domestic US deposits of some refractory minerals are limited, imports play a meaningful role in supply for these industries.

Clay is not a single commodity: kaolin (china clay) is used in paper coating and ceramics; bentonite serves drilling mud and foundry applications; fire clay is a refractory input; and common clays go into brick and tile. Each type may fall under a different HTS heading with distinct duty treatment, so the mineralogical identity of the clay must be established before classification.