Tregonning Hill is the westerly of two granite hills overlooking Mount's Bay in west Cornwall, United Kingdom, the other being Godolphin Hill. They are approximately 6 kilometres (3.7 mi) west of the town of Helston. The Plymouth chemist William Cookworthy mixed china stone with kaolin, mined from the hill to make Plymouth porcelain in 1768; which was the first time hard-paste porcelain was made in Britain. Part of the hill is designated as a Site of Special Scientific Interest (SSSI) and at the date of notification (1994) was the only known site of western rustwort (Marsupella profunda) in Great Britain.