Birds’ eggs have astonished naturalists for centuries due to their beautiful colours and patterns. However, unlike other colourful materials in nature, such as bird feathers and insect cuticles, only two pigments are considered major contributors to producing the full diversity of avian eggshell colours. We examined other factors that influence the visual appearance of eggshells. Tinamou eggs are well known for their extremely colourful and glossy appearance, but the underlying mechanisms responsible for this are unclear. Using experimental manipulations in conjunction with angle-resolved spectrophotometry, optical calculations, scanning electron microscopy, and atomic force microscopy, we show that the glossy appearance of tinamou eggshells is produced by an extremely smooth surface. We also reveal the presence of weak iridescence on eggs of the great tinamou (Tinamus major), an optical effect never previously documented for any bird egg, and present some interesting findings on the pigmentary-basis of tinamou eggshell coloration. We discuss the need for further exploration into the production mechanisms of colour, and other optical effects, in avian eggshells to better understand their function.