Summary John Harrison (1693–1776) is regarded as the father of chronometry. During his lifetime, he relentlessly pursued one of humankind's greatest and oldest challenges—that of finding the longitude at sea. In succeeding (according to the rules dictated by an Act of Parliament), he bequeathed to humankind the most accurate portable timekeeper the world had ever seen. It is a remarkable fact that his timekeeper, known today as H4, remains more accurate than the majority of expensive mechanical wristwatches manufactured today. Such (...) accuracy required novel approaches to address the various difficulties that befall all mechanical watches, and Harrison overcame many of these with his own innovations. The reduction or elimination of friction is one such problem with clocks and watches, and from an early age Harrison demonstrated his mastery in this subject. This is typified by his choice of woods in his early clocks, and in later clocks by his ‘grasshopper’ escapement. In the 1750s, Harrison's attention switched from clocks to watches, necessitating a hardwearing, low friction material to be found for the pallets in the escapement of his timekeepers. He found these properties in diamond, and in utilizing this to great effect in H4's escapement, he became one of the first people to use diamond as a high-tech material. This paper describes a scientific investigation into the diamond pallets of H4 using Raman microscopy, X-ray diffraction and optical microscopy, to elucidate why diamond was used rather than a more conventional jewel such as ruby, and to gain some insight into how Harrison might have achieved their unconventional morphology. From the evidence presented here, together with evidence collected from primary sources, it is shown that his use of diamond as a hard, low friction material was nothing other than extraordinary, and should be regarded in the same high esteem as his other technological gifts to the world. (shrink)