London Stone is a historic landmark that can be found in a public display case at 111 Cannon Street in the City of London. This ancient stone is an irregular block of oolitic limestone, measuring 53 × 43 × 30 cm (21″ × 17″ × 12″). It is a remnant of a much larger object that had stood for many centuries along the south side of the street.
Source - https://en.wikipedia.org/wiki/London_Stone