Cobalt is a light bluish, steel-grey metal that can be polished to a lustrous finish. It is brittle and not ductile unless alloyed with other metals. The fumes of metallic cobalt are odorless and black. Different cobalt compounds have different appearances and smells, as well as different dust and smoke colors. Cobalt has two allotropes, hexagonal and cubic, with hexagonal being more stable at room temperature than cubic. Cobalt metal in powder form burns in contact with pentflurane bromide, is incompatible with molten ammonium nitrate, zirconium hydrogen nitrate and strong oxidants and should be avoided. Powdered cobalt ignites spontaneously in air. Cobalt dissolves in dilute acid and dissolves into ultrafine powder in water.