It must be jet Jet is a precious and rare natural resource. Only produced in the northern mountainous area of Fuping County, Shaanxi Province. Its color is heavy and greasy, its texture is meticulous, it is black as ink, bright and lovely, and it is very famous. Its hardness is about Mohs4o, and it is known as a precious material for processing inkstones and knives. The ancients called it "gem", which can be divided into diamonds, precious stones and colored stones. Black jade refers to a valuable variety of Hetian jade in Xinjiang, which has the excellent quality of Hetian jade and is compact and moist. Among them, pure black jade is as black as ink, thick and greasy in color, delicate in texture, smooth and elegant.

There is also Feng Shui, which says that black is water and water is wealth.