Sources and production
Cobalt is found in Earth soils (typically 1-20mg/kg dry weight), and it is likely that it will be found in Martian soils. Mars is close to the asteroid belt, and so Mars has been peppered by metallic meteors, making trace amounts in Martian soils likely higher than on Earth.
As it is required in trace amounts for animal life, cobalt can be brought from Earth until the settlement(s) desire to become entirely self sufficient. (Likely in the form of vitamin pills.)
Cobalt is a transition metal. Cobalamin, or vitamin , has Cobalt at its active center. It is essential to all animal life. Lack of cobalt has caused deficiency sickness in a number of cases in Australia.
Cobalt is used in many high temperature alloys.
Cobalt can be used a catalysts in the petrochemical industries, and could be used to build petrochemicals from the carbon dioxide extracted from the Martian atmosphere.
Cobalt is used in some lithium ion batteries, but several modern battery cathodes dispense with it.