Stress marks may be seen on the tempered glass, which are usually due to the reflection under the polarized light, such as sunlight or other forms of reflected light.
The colors of the stress marks are sometimes referred to as iridescent, or the general condition as iridescence. Sometimes the stress mark looks like checkerboard or chain, etc., especially when you use polarized lenses to watch the glass.
Stress mark is characteristic of all fully tempered glass, which may become apparent under certain conditions of illuminations, especially when light is polarized. However, a well designed tempering furnace can maximally reduce the stress marks.