Tantalum is a hard, ductile heavy metal, which chemically is very similar to niobium. Like this, it easily forms a protective oxide layer, which makes it very corrosion-resistant. Its color is steel grey with a little touch of blue and purple. Most tantalum is used for small capacitors with high capacity, like those in cellphones. Because it is nontoxic and well compatible with the body, it is used in medicine for prostheses and instruments. Tantalum is the rarest stable element in the universe, however, Earth has large deposits. Tantalum carbide (TaC) and tantalum hafnium carbide (Ta4HfC5) are very hard and mechanically enduring.