Whitby jet is fossilised wood from the Jurassic forests that once covered the Yorkshire coast, washed out of the cliffs between Boulby and Robin Hood's Bay and worked into mourning jewellery since at least Roman times. The town has been the centre of the British jet trade for centuries.
A handful of family workshops on Church Street and Sandgate still cut, polish and carve local jet by hand — Hamond's, the Ebor Jetworks and W. Hamond among them. Look for the deeper black, slight warmth to the touch (unlike plastic imitations), and don't confuse it with the cheaper imported French jet. The Whitby Museum and the Captain Cook Museum both have good historical jet collections worth comparing before you buy.