Hearts and arrows is a style of cut used for round brilliant diamonds. Looking at the pattern is also a way of measuring optical symmetry, while gem laboratories only measure physical symmetry.
The Hearts and Arrows pattern appears in brilliant-cut diamonds that have very good parallelism and symmetry. When viewed from above, the "crown", the pattern is a series of eight gray arrowheads. From below, the "pavilion", the pattern appears as eight heart shapes. Hearts and Arrows viewers eliminate incoming light from certain angles and cause the pattern to appear black or very dark gray. In order to display this pattern flawlessly, the diamond's top facet or "table facet" must be exactly perpendicular to the bottom of the diamond or "pavilion". It must also be cut to have great symmetry of its major facets and have particular lengths of its lower girdle facets.
Diamonds with a Hearts and Arrows cut command a price premium in the world's market, reflecting the generally greater time needed to produce them and the greater loss of weight from rough, as well as their generally better overall cut quality. It has also become a popular sales tool in diamond marketing. Although the «Hearts and Arrows» property is indicative of a top-tier cut, it does not always mean the diamond will be the most brilliant. Optimal facet placement is the key to brilliance and more important than facet patterning. Some people may dislike the hearts and arrows patterning as well, since they may see dark arrow-shaped shadows in their diamond under certain lighting conditions.
While the major facets must be very symmetrical, there is considerable room for variation in cut quality and appearance from the "minor" facets, and from the angles of the major facets.
(Source: Wikipedia.org)
