Many people confuse diamond shape with diamond cut, one of the 4 C's of diamonds, but cut has to do with how the facets relate to one another and return light to your eyes, whereas shape refers to the outline of the diamond. Cut is what makes a diamond sparkle, shape is the external form. Here is a chart with images of the most common diamond shapes. The round brilliant is by far the most popular shape for diamonds, and it's the only diamond shape for which the GIA will give a cut grade--all other shapes are considered fancy shapes and don't get cut grades. There are general guidelines for desirable length to width ratios for fancy shapes, but their beauty is a bit more subjective and less quantifiable than the beauty of a round brilliant. While round brilliant diamonds never go out of style, some of the other fancy shapes go in and out of fashion over the years. Most recently, cushion and oval shapes have been the most popular fancy shapes for engagement rings. What's your favorite shape?