A limpet feeds by dragging a radula, a ribbon carrying rows of hardened teeth, across rock to scrape off algae. The teeth wear out from the front and are continuously replaced from behind, but while in service they must cut mineral without shattering.
Work published in 2015 measured the tensile strength of individual limpet teeth and reported values in the region of three to six and a half gigapascals. The structure responsible is a composite: fibres of goethite, an iron oxide-hydroxide, only a few tens of nanometres wide, embedded in a chitin matrix. Because the fibres are below the size at which flaws dominate failure, strength approaches the theoretical limit of the material rather than being set by defects.
Two qualifications matter. The measurement is tensile strength of a very small sample, not toughness or hardness of a whole structure, and the record is for a biological material tested this way, not for all natural structures under all loads. The scraping also removes rock, which is why limpet grazing measurably erodes shorelines over time.