Marquise Cut Diamond
The Marquise Cut is a fancy diamond shape that could be described as a "pointed oval". They are often mounted as solitaires in rings and may be set with the gem either pointing across the hand or in the same direction as the fingers. When worn with the gem running in the same direction as the fingers, Marquise diamonds are known for their flattering effect of making the hand and fingers look longer and more slender. The optimum length-to-width ratio of the Marquise Cut diamond is 2:1, with between 1.75:1 and 2.25:1 being considered acceptable.
Legend has it that the Marquise was originally commissioned by Louis XV of France, the "Roi du Soleil" or Sun King, who wished to created a diamond in the shape of the mouth of his mistress, the intelligent, beautiful and refined Jeanne-Antoinette Poisson, Marquise de Pompadour.  The Marquise Cut is also sometimes known as the Navette Cut - and "Navette" means "Little Boat".
Purchasing A Marquise Cut Diamond
As with other "pointed" diamond shapes, it is advisable that Marquise Cut stones be mounted in a manner that protects the more delicate points from damage. Also, care should be taken to see if the Marquise has flaws near the points as these could increase the risk of a breakage at that point. The Marquise cut gem is said to be among the most difficult gems to set properly and should not be structurally supported only by the tips in its setting. 
Another point for the prospective buyer is to be aware that a Marquise that is too thin or not well cut, may have a darker "bow tie effect" (a dark patch that looks like a bow tie. All Marquise Cut diamonds (and many other fancy shaped diamonds) will have some level of bow tie, even if the stone is immaculately cut - however the noticeability of the effect varies from stone to stone. 
The GIA (Gemological Institute of America) has not published specifics as to what should constitute an "ideal cut" for the Marquise Cut diamond. Due to the fact that the length to width ratio is variable in the Marquise, ideal proportions are harder to establish. However, it has generally been agreed upon by cutters and gemologists that a depth percentage (this measurement is obtained by dividing the diameter by the depth) of between 63% and 68% will give the greatest brilliance while minimizing the bow tie effect.  The stone should also have good symmetry.
Marquise Cut Diamond in ring
There also appears to be no set number of facets to a Marquise Cut. It didn't take me long researching this article to find different facet diagrams and statements that disagree: Stating that a Marquise cut "is" cut with one of a number of variations; including 57 (58 with culet) - with 33 crown and 24 pavilion facets; and 55 (56 with culet) - with 33 crown and 22 pavilion facets.
Marquise Diamond facet diagram:
Above - with "French tips". Below - normal.
Also, an interesting modification to the Marquise Cut is to cut the points in the form of what are called "French tips". These are said to make the points less fragile, however I have also read that the additional number of facets may make it harder to determine clarity in the critical area of the points.   In the facet diagram to the left, the upper example (with French tips) has 31 crown facets, the lower has 33 crown / 24 pavilion.
Marquise Cut diamonds have a long history of popularity and celebrity wearers include: Traci Bingham; and Catherine Zeta Jones - who was famously presented with a 10 carat antique Marquise diamond engagement ring by Michael Douglas.
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Note - this website is intended for general informational and entertainment purposes, and should not be considered to be professional consultation. If you are considering purchasing precious stones, be sure to seek the advice of a qualified professional.