Jewelry Fashion, like clothing fashion follows certain trends depending on the times. Just as "mini skirts were once fashionable, so were certain diamond shapes & cuts. At one time, the marquis shaped diamond was most popular. However, the shape for a diamond that has remained the most popular for centuries is the round style.
Aside from Clarity, carat weight & color, perhaps the most important to determine the diamonds sparkle is how it is cut. Light Passes through the top or diamond table and is refrected at various angles to produce its sparkle and quality.
In the 19th and early part of the 20th century a "miner's cut" became popular. This was acheived by cutting the point of the diamond to fit snugly into certain settings. This process however, reduced the refraction of light or the diamonds sparkles. Therefore in today's diamond market, miners cut stone are less desirable and hence, less valuable.