Jet

Jet is coal — specifically it is lignite, the softest and youngest rank of coal, formed from wood rather than from peat swamp and compressed without much heat.

What makes gem jet different from the lignite in a power station is the history of the particular wood. The Whitby material began as driftwood from araucarian conifers that floated out to sea, waterlogged, sank into anoxic seafloor mud, and was buried in what is now Jurassic shale. Marine burial rather than swamp burial is what produced a compact, uniform, structureless solid instead of a banded and friable one.

So it takes a polish that no other coal will take, holds a carved detail, and is very light — which is the whole of its appeal as a material. A full mourning parure in anything denser would have been unwearable.

The Whitby jet beds sit in the Toarcian shales of the Yorkshire coast, around 182 million years old, and the sequence that made them is unusually well understood because the geology records each step.

Trees fell into rivers and were carried to sea. Waterlogged trunks sank rather than beached. The seafloor at that time and place was anoxic — the same condition that makes the surrounding shale black and organic-rich — so the wood was not consumed by anything. Sediment buried it, compaction flattened it, and the low temperature of shallow burial carried the coalification only as far as lignite rather than on to bituminous coal or anthracite.

Every one of those conditions matters. Jet is rare because the sequence is, not because the material is exotic.

Jet has been imitated for as long as it has been fashionable, and the substitutes are all denser, colder or shinier in the wrong way.

Weight is the fastest test. Jet is about 1.3 g/cm³ and warms to the hand almost immediately. Black glass — 'French jet' — is more than twice as dense and stays cold.

The streak is the definitive one. Rubbed on unglazed porcelain, jet leaves a brown streak. Glass leaves none, and the black plastics leave none.

A hot point distinguishes the Victorian organic imitations: vulcanite, which is heavily vulcanised rubber, smells of sulfur, and bog oak and horn smell of what they are. Vulcanite also fades to a khaki brown in sunlight, so a 'jet' brooch that has gone dull olive is a vulcanite one.

And like amber, jet takes a static charge when rubbed and will lift a fragment of paper — which is where 'black amber' came from, and it is a shared behaviour rather than any kind of relationship.

Jet has been carved since the Neolithic — Whitby jet appears in British Bronze Age burials, and Roman Britain worked it at York into pendants and hairpins, some of which travelled to the Rhineland.

Its modern significance is almost entirely one decade of one century. Prince Albert died in 1861, Victoria wore mourning for the remaining forty years of her life, and the elaborate British etiquette of mourning dress required jewellery that was black and not showy. Jet was the only material that qualified, and Whitby went from a handful of workshops to around fourteen hundred people employed in the trade by the 1870s.

The fashion ended with the century and the industry collapsed with it — a clean example of a material whose entire market was a social convention, and which had nothing else to be when the convention passed.