Geologically, the occurrence of indium minerals is rare. The element most often occurs as a sulfide inclusion or substitutes in other base-metal minerals, including cassiterite, chalcopyrite, sphalerite and stannite. Indium’s abundance in the crust is estimated to be 0.05 parts per million, which makes it more abundant than silver, but it is so widely disseminated that it does not occur in high enough concentrations to form mineable deposits. Therefore, indium is most often recovered from byproduct residues produced during the refining of lead and zinc. But only about one-quarter of the indium mined worldwide is refined into metal, as many indium-bearing concentrates are sent to refineries that do not have the capability of recovering the metal.