Definition: black or occasionally very deeply colored red glass, often backed with metal foil, that's glued, fused or soldered onto a metal setting; originally intended to imitate jet, though it possesses a harder, darker glitter than the real thing; French jet beads are sometimes roughly faceted to look like jet, but will be heavier in weight and cold to the touch when compared to genuine jet; developed in the early 19th century and perfected in the 1860s, it was manufactured in France - hence the name - though also in Austria, England, Germany, and what is now the Czech Republic
Example: Because it was much cheaper to produce than authentic jet, French jet became the premier source of modestly-priced mourning jewelry.