Cobalt looks like a silver-and-grey tinted metal. When combined with other metals to form an alloy, it can become different shades of blue, yellow and green.
When cobalt is combined with aluminum, the alloy is referred to as cobalt blue. Alloys of cobalt and zinc are green in color. An alloy of cobalt and potassium is tinted yellow or gold. These alloys are often used to color glass, enamels, tile and other forms of pottery. Smalt, a substance containing cobalt, has been used to create blue glass since the Middle Ages. Cobalt is also used to create paints and dyes.