It is byssus, a type of natural marine silk also called “sea silk”. It is obtained from the solidified filaments of Pinna Nobilis Setacea, a mollusk similar to a large mussel that can reach over a meter in length. It produces a brown gelatinous filament that solidifies on contact with water and allows the mollusk to anchor itself to the seabed. The filament, much thinner than a human hair but at the same time infinitely more resistant, is cleaned, combed, twisted and used for textile processing.