pavillion (bottom) view, side view and table (top) view of a brilliant cut
Each facet on a stone is involved in drawing light into the stone and reflecting it back out again. The modern brilliant cut diamond is the ideal cut that if cut to perfect angles and proportions will display a stone's ultimate lustre, fire and brilliance by reflecting and refracting the light that travels through it perfectly. If a stone is cut too deep or shallow, light will be lost and the stone will appear dull, or light will be reflected at the wrong angles causing unwanted patterns and shadows within the stone.
the journey of light in a perfectly cut stone
The nature and identification of a stone would dictate the cut. Some cuts are specific to stones that can reflect light ie: crystals. Where other simpler cuts are used for those that cannot reflect light. The durability and hardness of a stone would also dictate the cut.
Each specific cut has it's own pleasing characteristics, some are more delicate than others. Mostly any cut with a corner needs to be considered fragile. Also keep an eye out for any girdles on the side edges of a stone before you buy it. If a girdle is cut too sharp it is liable to chip easily, try ensure that a girdle facet is present between the two bezels ( crown and pavilion ) Diamonds can have either a polished (prefered) or occasionally an unpolished girdle. Every stone is governed by it's cut when it has to be set in metal. Not all stones can be set the same. Exposing too much of a stones girdle is not personally recommended as it is allowing the risk chipping the stone especially if the stone has a sharp girdle.
The cut of a diamond is probably one of the most detailed grades in a certification Depending on the source of the certification, careful analysis on each angle of the table, crown and pavilion can be noted, as well as the overall symmetry and proportion of the table, crown and pavillion. Even the stones overall polish is included as being part of the cut. Some diamond certifications offer information about another interesting feature about the diamond brilliant cut, namely the presence of Hearts and Arrows.
Heart and Arrow shapes occur in a diamond which has been cut to the ideal brilliant cut proportions. There are 8 heart shapes that occur when a diamond is viewed from the pavillion view. There are 8 arrows that can be seen from the table view. Grades are given according to the accuracy and proportions of these shapes.
Pavillion View Table View
This is just one more instance that shows how a cut can influence overall performance of a stones play on light. Not only diamonds display this characteristic and the presence of hearts and arrows is by no means a measure on the quality of a stone, but more on the quality grade of the stones cut and proportions in relevance to the modern brilliant cut.