Jet is a tough, compact black form of lignite, or brown coal, and is produced by plant decomposition and fossilization. It is lightweight and takes a very high surface polish. It is also soft enough to carve directly with a metal blade, bringing it within the range of possible materials for the scholar-carver to express his own artistry with the "iron-brush" of the seal-carver. Jet was also an appealing material because of its association with longevity, since one popular Chinese misconception was that it took amber a thousand years to form, and that after a thousand more amber became jet. Its relative rarity prompted imitations in lacquer or black glass.
The faceted design reflects the popularity of faceting at the court, introduced by the Bavarian Jesuit Kilian Stumpf in the late-seventeenth century and used to decorate a wide range of Imperial snuff bottles during the Qing dynasty.