“Jack” is the slang term or nickname for the mineral sphalerite (ZnS).  Sphalerite is the main ore mineral for the metal zinc.

The name Jack is probably derived from “jaune,” the French word for yellow.  Sphalerite is often a golden yellow or yellowish brown in color.  Some varieties are very dark brown to black – called black jack – or are a translucent red – called ruby jack.

Missouri was a major producer of sphalerite, especially during the heyday of the Tri-State District, located in southwest Missouri, southeast Kansas and northeast Oklahoma.  The mining district was originally known for production of galena, a lead ore.  However, improved processing of the ores led to zinc becoming the dominant metal product in the early 1870s.  The region became so well known for the mining of sphalerite, or jack, that the town of Joplin later became known as "the town that 'jack' built."

Missouri continues to produce sphalerite from mines located in the Viburnum Trend, located in southeast Missouri.