Hearts and Arrows is an industry term for a specific type of round cutwhere all facets are in perfect alignment, proportion and symmetry. Cut is one of the four major components of diamond grading.  Cut controls 98% of the brilliance of a round cut gem or diamond. However, cut is often the least understood of the four C's, and often the most disregarded. It's easy to understand carat weight, color and clarity, but cut is difficult.

How the  facets on both the top and bottom half of the round cut gem line up (symmetry) determines a gemstones scintillation, or the way it plays with light as it reflects it to the viewer.

The interplay of all of these facets on both the top and bottom half of the diamond or diamond simulant control how much light that enters a the stone is returned back to the viewer.  The inside of a diamond can be represented as a hall of mirrors.  The more precisely aligned and shaped the mirrors, the more light will be reflected back out rather thanlost as it bounces between mirrors. In a typical diamond, roughly 68% of the light that enters ends up being reflected back to the viewer. 

By contrast, in a hearts and arrows diamond, 96-98% of all light that enters is reflected back out!  This is a result of a painstaking effort on the part of the cutter to ensure that all facets (mirrors) are perfectly symmetrical, in perfect alignment and perfectly proportioned. It is very easy to see the difference in diamond when comparing a
typical round diamond to a hearts and arrows diamond. The difference is simply a much more brilliant stone!