Cobalt is a metallic chemical element (atomic number 27). It was the first metal to be discovered and isolated in modern times. Like iron and nickle, it is magnetic and, when alloyed to other metals, retains it's magnetic properties. It also resists wear and greatly improves the strength of an alloy.
In medicine, cobalt is used in high-wear high-strength environments, such as hip joints. It's radioactive form is used in radiation therapy. It is also a trace nutrient, being a vital component of Vitamin B-12. However, larger dose are toxic. Cobalt poisoning is not uncommon in artists, where cobalt salts are used to make intense blue pigments.