(photo courtesy of John David McFarland)
The photo above illustrates surface iridescence. Iridescence is a play of colors caused by the interference of light waves. This phenomenon is also called thin film interference. You have probably seen this effect on soap bubbles and oil sheens. Light reflecting from a thin coating of iron oxide on the piece of novaculite above is producing the play of colors. Light waves are reflected from the top of the iron coating and the base of the iron coating producing multiple waves. A color is seen when the waves interfere constructively. The resultant color is dependent on the thickness of the coating and consequently, streaks and bands of differing color develop since the thickness of the iron oxide coating varies. The colors also change when the angle of reflection is changed.