Jet is a black organic rock that forms when pieces of woody material are buried in sediment and are coalified. Though very similar to coal, it is less friable. Jet can be cut, carved, and polished to a bright luster. People have used jet for thousands of years to produce gemstones, beads, and many other objects. Jet is one of just a few organic gemstones. It is the material that inspired the phrase “jet black,” which means “as black as possible.”

Faceted jet: Four stones of faceted jet from the Whitby area of England. These stones clearly show how jet can accept a highly reflective polish. The round stone at the bottom of this photo is about 12 millimeters in diameter.