The prime function of jets seems to have been to signify that pilgrims had completed their journeys, and reached the shrine of St James (the patron Saint of Spain) at the Cathedral of Santiago de Compostela, which pilgrims did over the course of six hundred years, from the twelfth to the eighteenth century.
In structure jet is a particularly dense type of coal, and can be carved and polished. Medicinal and indeed magical qualities were thought to be inherent in the substance from earliest times. Jet is found in different parts of Europe (as well as North America), but the two richest regions are the Asturias in northern Spain, and Whitby, Yorkshire, in North East England.