Before its Victorian mourning association, jet already carried a long-standing protective reputation in Britain and beyond, worn as an amulet against the evil eye and against illness. Roman-period grave goods across Britain include jet amulets, and archaeologists have proposed that the material's deep black color and ability to take an electrostatic charge when rubbed, a property it shares with amber, contributed to a folk sense that jet held some active power rather than being simply decorative.
Outside Britain, jet from deposits in Spain, particularly around Santiago de Compostela, was carved into pilgrim badges and amulets associated with the shrine of Saint James, giving Spanish jet its own separate devotional history running in parallel to, rather than derived from, the English material's.
The Roman name for the material, gagates, is where the English word ultimately comes from, and Pliny the Elder derives it from Gagas, a river and town in Lycia in Asia Minor. Pliny also records what makes jet strange among black stones: it burns, and burning it gives off a strong and distinctive smell, which he says was used both to drive off snakes and as a test applied to someone suspected of epilepsy. Whatever those procedures actually accomplished, the underlying observation is sound and unusual for a gem material, and it goes a long way towards explaining why jet accumulated a reputation for holding some active property rather than for being merely black and polishable.