A monocrystalline whisker is a filament of material that is structured as a single, defect-free crystal. Typical whisker materials are graphite, alumina, iron, or silicon. Single-crystal whiskers of these (and some other) materials are noted for having very high tensile strength (on the order of 10–20 GPa). Whiskers are used in some composites, but large-scale fabrication of defect-free whiskers is very difficult.
Prior to the discovery of carbon nanotubes, single-crystal whiskers had the highest tensile strength of any materials known, and were featured regularly in science fiction as materials for fabrication of space elevators, arcologies, and other large structures.