When a lab like GIA assess the 'cut grade' of a diamond, they measure an average of the main facets on the crown and pavilion of the diamond. This method means that, whilst many facets on the diamond might not be correctly positioned, as long as the averages fall within certain ranges, the diamond can still be Excellent Cut.
Hearts and Arrows looks at the optical symmetry of a diamond, i.e. making sure that every group of facets is at the same angle on a round brilliant cut diamond. The term Hearts and Arrows comes from the pattern displayed on a perfectly cut stone, with perfect internal optical symmetry, when viewed under a special viewer.
When you look at the stone from above through the viewer, you will see a series of 8 perfectly formed arrows on a standard 57 facet round brilliant diamond. When you view the stone from the bottom, you will see 8 perfectly formed hearts. There is no difference in terms of the number of facets - it means though that every facet is cut to the same angle creating perfect optical symmetry.
A common misconception is that a diamond that is known as a ‘Triple Excellent’, which means it has Excellent Cut, Excellent Polish and Excellent Symmetry, will also display a Hearts and Arrows pattern. Unfortunately, this is not the case. Because the cut grade is based on an average of the main angles, the position of the facets can vary quite significantly.
You can see the differences in the optical performance of a diamond which is perfectly cut with Hearts and Arrows versus an Excellent Cut without Hearts and Arrows. In this comparison, the Hearts and Arrows diamond on the left has more sparkle evenly distributed when compared to the Excellent Cut stone without Hearts and Arrows.
Comparing the light return of the same two diamonds, you can see that the Hearts and Arrows diamond is noticeably brighter. It also has a better contrast pattern.
Comparing the levels of 'fire', or light refraction, of the two diamonds, the Hearts and Arrows stone has more fire that is evenly distributed across the stone.
