Sea silk may sound like the stuff of myths and legends along with mermaids and sirens, yet actually lustrous sea silk is harvested from a rare species of clam, whose delicate fibres flash gold in the sunlight.

Elusive strands of golden sea silk are produced by the Pinna nobilis, otherwise named the pen shell a giant Mediterranean mollusc that can measure over a metre in length. In order to attach themselves to rocks these large clams secrete proteins that, upon contact with seawater, form into threads called byssus. The byssus of the pen shell makes sea silk, the world’s most sought after and rare golden thread. Up to three times finer than human hair the sea silk has an ethereal golden sheen.

It is believed that only one person alive knows how to spin this natural material into golden thread. On a little island near Sardinia, Chiara Vigo who is in her mid-sixties dives up to 17 metres deep, into a complex of secluded submerged caves that the women in her family have kept secret for the past 24 generations. Her moonlight dives take place in the spring where she uses a tiny scalpel to carefully trim the razor-thin fibres growing from the tips of a highly endangered Mediterranean clam.

As the Italian Coast Guard survey her from the shore, she may dive multiple times to produce just a handful of fibres. She gently trims away the byssus from each bivalve. Vigo says that her un-intrusive method, is just like giving the clams a haircut and does not harm them in any way. She says a prayer before each dive, and lives by an ancient, sacred ‘sea oath’ that prevents the sea silk from being neither bought nor sold.

The Latin term ‘byssus’, can be found in many ancient documents, and appears to refer to both fine linen and also sea silk. Researchers have asked if this was in fact a holy material used in church textiles. One beautifully crafted stocking, spun from sea silk and dating from the 17th century, is now on display at the Naturhistorisches Museum in Braunschweig, Germany.

In the past, people often thought this subtle thread came from the mythical water sheep. These fictional beasts appear in stories from Chinese silk traders dating back to the second and third centuries. Water sheep, with duck-like webbed feet and golden fleece, were said to live beneath the waves of the Roman Empire and similar stories feature in the records of 13th-century Arab traders.

While Chiara goes about her arduous sea silk collection process, she refuses any payments for her lovingly gathered threads, only sometimes she supplies it for use in embroidery of a child’s christening gown, and only occasionally to grander scale textiles.

Sadly, if the decline in sea water quality continues to dive, then alchemic sea silk and its giant hosts may disappear forever, and the art of sea silk weaving may be lost into the dark depths of the history books.