Wheal Castle appears to have not been very successful, and eventually was used just as a quarry for roadstone.
1821 Joseph Carne, writing for the Royal Geological Society of Cornwall in 1821 said of Huel Castle Copper Mine. This mine has been wrought in a very limited way. The extent which has been explored under the sea does not exceed ten fathoms, and in that space nothing, has occurred worthy of particular remark.
1856 Kellys Directory. Boswedden and Wheal Castle Mine is in the parish of St. Just, union of Penzance, hundred of Penwith, Cornwall, and within the mining district of St. Just.
1873 Kellys. Wheal Cunning United mine includes what were formerly Boswedden and Wheal Castle. The Wheal Castle portion has been explored for a distance of half-a-mile under the sea. There are three pumping engines, two stamping engines, and two drawing engines: there is also very great water power on this mine, which is used for stamping and drawing purposes.
1884 The sale of roadstone probably commenced here in about 1884. Initially this was from rock broken underground but proper quarrying commenced soon after. Rock was to be carried by ship, so loading facilities were constructed around Wheal Castle Zawn. It is difficult to know how much rock was sold, as well as iron ore, but the first ship was loaded on 8 March 1886 and continued until Wheal Castle closed the following year. A loading plat was constructed above the Zawn and a steam crane was acquired from a quarry near Helston, erected on the west side of the Zawn.
1910 the two quarries on the headland were opened to provide stone for the
roads of West Penwith. A narrow-gauge tramway was built to remove the stone,
and the quarries worked until 1921 although suspended for the duration of the
St Just Area Mines, Cornwall