Look closely at a piece of jet and it doesn’t look particularly impressive at first. Fresh from the cliff, it is not the deep, glossy black that we associate with Victorian jewellery. It is often brownish-grey, dull, and easy to mistake for an ordinary lump of coal. But polish it, and something remarkable happens. The surface transforms into a deep, velvet black that catches the light like glass. For centuries, this strange little piece of fossilised woodland has been pulled