Semi-precious materials such as this often associated with a particular geographical region and highly valued because of their rarity. Many of the religious objects were portable, as were the small portraits. They were often made near to the source of the material and then taken elsewhere.
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.
The iconography of this piece implies it derives from seventeenth-century painting and sculpture. It could represent the Virgin of Guadalupe, devotion to which was propagated at Santiago by the Archbishop Monroy (b. 1934; archbishop 1686-1715), who had been born in Mexico.
The original function is uncertain. Perhaps it was simply displayed and handled like a medal.