Glassworker is a machine added Extra Trees. It can be used to form a block of stained glass with different color patterns.
You need 2 blocks of Stained Glass or Pigmented Glass (Botany) and Glass Fittings (Extra Trees) as an adhesive (or Wood Polish (Extra Trees) for old versions). The stained glass goes into top slots of the machine (order matters), the adhesive on the bottom. Then you can preview one of the many patterns by clicking on them. The menu is scrollable and filterable by name (enter text on the bottom right).