Cobalt
Cobalt is a silver-coloured metal that gives a blue colour to things like (Chinese) porcelain and glass. Cobalt is a mineral but also occurs naturally in the human body (it's a component of vitamin B12, which occurs naturally in the human body — meaning the body contains roughly 1 to 2 grams of cobalt). Cobalt is also often found in meteorites, together with nickel and iron. Cobalt is generally used in the manufacture of metal joints (think of steel pins for bone fractures), metal products such as buckles and tools (e.g. scissors), magnets and batteries, and for that reason is often used alongside nickel and iron. It is also frequently found in cement, hair dyes (particularly in brown tones) and printing ink.
You'll recognise cobalt on the ingredients list by the symbol Co. Cobalt chloride or cobalt dichloride is a chloride compound of cobalt (it's pink and powdery) and so is often mentioned in the same breath as cobalt. You'll recognise cobalt chloride or cobalt dichloride on the ingredients list by the symbol CoCl2.
Cobalt and cobalt (di)chloride are known as allergenic substances; they carry a high risk of skin allergy, especially when used together with nickel. Note, however, that an allergy to cobalt (or cobalt (di)chloride) alone is rare. Avoiding the substance cobalt (or cobalt (di)chloride) is almost never the solution for a cobalt allergy. People with a cobalt allergy are usually (in 75% of cases) also allergic to nickel and/or chromium (nickel, chromium and cobalt are often used together when making metal alloys). When a cobalt allergy is present, there's usually also a nickel and/or chromium allergy. It's these last two substances in particular that cause most of the complaints in an allergy.