An Iridescent “Insect Wing” in Diamond
Diamond is very resistant to scratching and is often referred to as the hardest mineral on Earth due to its compact crystal structure. However, diamond is not indestructible, and it can be fractured or even broken apart during ascent from the mantle to the earth’s surface through fast, violent kimberlite eruption. Collision with rocks in high-energy rivers also results in percussion marks on the surface of alluvial diamonds (e.g., J.W. Harris et al., “Morphology of monocrystalline diamond and its inclusions,” Reviews in Mineralogy and Geochemistry, Vol. 88, No. 1, 2022, pp. 119–166). Surface cracks extending into the interior of a diamond are referred to as “feathers” in the gem trade, and these often have a negative effect on the clarity grade.
Fractures within diamond can cause fascinating optical phenomena on rare occasion. The authors recently examined a 0.39 ct Fancy Deep brownish yellowish orange type Ib/IaA diamond containing multiple feathers and graded as I2 clarity. These natural features had not been filled with a clarity-enhancing material. The largest feather located on the table facet of the diamond resembled an iridescent insect wing (see above).
Iridescence is an optical phenomenon caused by interference of light, typically seen in gemstones with cleavage or repeating submicroscopic structures (e.g., X. Lin and P.J. Heaney, “Causes of iridescence in natural quartz,” Spring 2017 G&G, pp. 68–81). In this diamond, the feather’s iridescence was due to thin-film interference caused by a thin film of air with a thickness similar to the wavelength range of visible light. As diamond and air have very different refractive indices, there is a phase difference between the incident light reflected from the upper and lower boundaries of the thin film, resulting in constructive and destructive interference. When viewed using a broadband light source (light consisting of a wide range of wavelengths), constructive or destructive interference intensifies or attenuates certain wavelengths (colors), respectively, producing a rainbow-like interference pattern.
