The cable for a space elevator must be made of a material with an extremely high tensile strength/density ratio (the stress a material can be subjected to without breaking divided by its density). A space elevator can be made relatively economically feasible if a cable with a density similar to graphite and a tensile strength of ~65-120 GPa can be produced in bulk at a reasonable price. By comparison, most steel has a tensile strength of under 1 GPa, and the strongest steels no more than 5 GPa, but steel is heavy. The much lighter material Kevlar has a tensile strength of 2.6-4.1 GPa, while quartz fiber can reach upwards of 20 GPa; the tensile strength of diamond filaments would theoretically be minimally higher. Carbon nanotubes appear to have a theoretical tensile strength and density that is well above the desired minimum for space elevator structures. The technology to manufacture bulk quantities of this material and fabricate them into a cable is in early stages of development.