Mining also is no longer allowed along the craggy cliffs of Whitby, although the beach often is crowded with rockhounds, amateur stone hunters who comb the coast for jet, a mineraloid that is the fossilized remains of an ancestor of the monkey tree.
But blue john, a semiprecious mineral that is a variety of fluorite with distinctive blue-purple and yellow-white banding, has been mined in Castleton, in the Peak District, since the early 18th century and possibly earlier, Ms. Hansen said.
Chris Sellors is the owner of C.W. Sellors Fine Jewelry, which has 19 stores across northern England, including in Castleton and Whitby, selling jet and blue john jewelry. He said the only remaining blue john mine, which was in Castleton, was restricted to producing a half a ton of material a year.
During the material’s 18th-century heyday, it could be found in large quantities and was used extensively in ornamental objects. According to Mr. Sellors, a manufacturer named Matthew Boulton, whose highly decorated gilt and blue john pieces are now in the Royal Collection at Buckingham Palace, managed to acquire 17 tons of blue john in 1870 alone.
Today, as blue john is found only in small deposits, the delicate stone is well-suited to jewelry once it has been strengthened with a mother-of-pearl backing, Mr. Sellors said.
With his establishment of the W. Hamond Museum of Whitby Jet in 2019 and annual travels to the gem shows at Tucson, Mr. Sellors is trying to promote British gemstones beyond tourist trinkets. “It’s a small market, but we’re very proud of what we do,” he said. “We want to elevate it as a material. Tanzanite is very rare, but blue john is much rarer, and so is jet.”