Silicon, germanium, and tin, below carbon in Group IVA, all have the diamond structure but with increasing gray color and metallic luster. Tin also has a second, metallic allotrope. Gray tin, which has the nonmetallic diamond structure, is the stable form below 13°C, and metallic white tin is more stable above this temperature, although the rate of conversion from one to the other is very slow. These allotropes led to considerable grief for medieval metallurgists, who noticed every so often that tin objects left out in the cold would begin to decay in spots and crumble into a nonmetallic gray powder. They thought it was a disease of the metal, and called it "tin pest." Actually, it was only the slow conversion of white tin to gray tin at low temperatures. Because the change is slow, we ordinarily do not observe it; the tin remains metastable in the metallic form. Lead, at the bottom of Group IVA, occurs only in the metallic cp structure.