Great question! You could, but it is very uncommon. We’ve seen low grade, cheap black and gray cabochon cut diamonds. Cabochons are very common in colored gemstones. Ruby, Sapphire, and Emerald are all (much) softer than diamond and therefore, they can be easily manipulated and fashioned into a smooth, domed shape – using a diamond-dust impregnated grinding wheel. However, this method cannot be applied with a diamond. The process of cutting a cabochon on a polishing wheel is long and tedious. Any experiments to polish a diamond into a cabochon shape using laser have failed miserably. In any case, the consensus is that a cabochon cut, gem quality diamond would look as attractive as a low quality piece of quartz. So why waste – and basically destroy – the material in the first place?