Organic
Fossilised driftwood compressed into a dense, velvet-black coal that takes a mirror polish and weighs almost nothing in the hand. Whitby jet, formed from Jurassic monkey-puzzle wood, is the historical standard and has been carved in Yorkshire for two thousand years. Hard jet is compact and glassy; soft jet cracks with age.
Real jet is warm to the touch, extremely light for its size, and shows a brown streak on unglazed porcelain. It takes a deep lustre without looking plasticky. Black glass ('French jet') is cold and heavy; vulcanite fades brown and smells of rubber when rubbed.
Never use solvents, ultrasonic cleaners or heat. Wipe with a dry soft cloth; a trace of natural wax restores lustre.
Jet was the mourning stone of Victorian England after Prince Albert's death, and Roman writers already recorded burning it to ward off illness. Pilgrim crosses of Spanish azabache were carved at Santiago de Compostela.
Cultural history, recorded as tradition rather than as medical or scientific claim.
See it in person
Both rooms are open six days a week, and there is always someone happy to pull pieces out of the case so you can compare them under the light.