The iridescence which forms on glass is caused by weathering on the surface of the vessel. How much iridescence is affected by a number of factors, including the humidity and type of soil and to an extent, the composition of the glass itself. Thin layers of weathered glass accumulate and then disperse (sometimes by flaking off) and display a refraction of light.
To find out more about Roman glass please see our relevant blog post: Collecting Roman Glass.