Tantalum is a gray, hard, rare, metallic element occurring in columbite and tantalite and usually associated with niobium: used, because of its resistance to corrosion by most acids, for chemical, dental, and surgical instruments and apparatus. Columbite forms a series with the mineral tantalite. In fact the two are often grouped together as a semi-singular mineral called columbite-tantalite in many mineral guides. The minerals of the Columbite/Tantalite families are difficult to distinguish from each other without sophisticated tests. The combination is also called Coltan; short for columbite–tantalite and known industrially as tantalite.