Tantalum is a chemical element with symbol Ta and atomic number 73. Previously known as tantalium, its name comes from Tantalus, an antihero from Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion-resistant. It is part of the refractory metals group, which are widely used as minor components in alloys. The chemical inertness of tantalum makes it a valuable substance for laboratory equipment and a substitute for platinum. Tantalum is also used for medical implants and bone repair. Its main use today is in tantalum capacitors in electronic equipment such as mobile phones, DVD players, video game systems and computers. Tantalum, always together with the chemically similar niobium, occurs in the minerals tantalite, columbite and coltan (a mix of columbite and tantalite). Tantalum is a rare metal, comprising 8×10−9% of the universe, making it one-fifteenth as abundant in the universe as gold (which makes up 6×10−8%). Tantalum also comprises 1.5×10−4% of the earth's crust, making it more abundant than other metals in the sixth period, such as rhenium (abundance 2.6×10−7%), osmium (abundance 1.8×10−7%), and iridium (abundance 4×10−8%), but not as abundant as barium (abundance 3.4×10−2%).