Metallic stones in glass are difficult, if not impossible, to identify by visual examination alone. Although metals such as iron and copper are the most common, reaction with the glass can create different sheens that alter the apparent color. On top of that, less common metals occasionally crop up, such as this nickel sulfide stone. Nickel sulfide is well-known in the flat glass industry as a cause of breakage for thermally tempered windows. This stone, however, was found in a glass bottle, and was caused by contamination of the internal cullet supply.