If I were you I would go to Diamond Source of Virginia http://www.diamondsourceva.com/Education/Shape/Shape.asp
to purchase the center stone to get a great stone at a great deal. They will find you a stone from what's on the market rather than from what they only have in their particular inventory. Therefore you would have access to a much larger selection. Their prices are also less than wholesale! Check out their calculator tool. You can call them or visit them in person. Then I would go to either Jared's or a local store to find and purchase a setting for your center stone.
I also suggest checking out their site for information about purchasing diamonds: http://www.diamondsourceva.com/Education/Shape/Shape.asp
It gives you info. on shape, cut, polish, symmetry, color, clarity, carat, cost, and certification. These are what you need to know in order to purchase a good quality diamond.
SHAPES: The princess, cushion, radiant shapes will be cheaper than the round shape of equal weight. Round shapes are more popular because they offer superior brilliance when cut well compared to other shapes. Therefore, there's more demand for them, and therefore, they are more expensive. However, the cushion and radiant shape will look bigger than their actual weight, while the round and princess shapes appear true to their weights.
CUT: Basically, after you've decided what shape diamond you want to purchase, the next thing to do is find a diamond with a great cut. The smaller the stone, the better the cut should be in order to maximize brilliance. Smaller stones are less than a carat and larger stones are stones more than a carat. You find out about the cut of the diamond from the certification information. Be sure to get either a GIA or AGS certification. Those are the best certifications. When searching for a great cut, the two key numbers are the depth percentage and the table percentage.
Depth percentage is the depth of the diamond (table to culet) divided by the width of the diamond. The higher the depth percentage, the deeper the stone looks in appearance. The lower the depth percentage the shallower the stone looks. For example, a diamond with depth of 4.09 mm and width (average diameter) of 6.75 mm equals a depth percentage of 60.6%. For a round diamond, a good target for the depth percentage is 59-62.5%.
Table percentage is the width of the table divided by the width of the diamond. The higher the number, the bigger the table looks. The lower the number, the smaller the table looks. For example, a diamond with table of 3.91 mm and width (average diameter) of 6.75 mm equals a table percentage of 57%. For a round diamond, a good target for the table percentage is 53-59%. In general, you want the bigger size diameter and the smaller table percentage for the most beautiful round diamonds.
Out of Round: It is interesting to note that "round" diamonds are usually not perfect circles. The length and width measurements for a round are both diameter measurements and will be different for an out-of-round shape. If the length is greater than the width by more than .10 millimeters, the diamond has not been cut well and should be avoided. For two-carat stones, the acceptable deviation is 0.12 millimeters.
POLISH: The polish grade is a measure of the visibility of polishing lines on the surface of the diamond when viewed with 10X magnification or the unaided eye. For round brilliant and fancy-shaped diamonds, a polish grade of Good or better is generally preferred.
SYMMETRY: Symmetry refers to the external balance and alignment of the facets. For round brilliant diamonds, a symmetry grade of Very Good or better is generally preferred. For fancy-shaped diamonds, a symmetry grade of Good or better is generally preferred.
GIRDLE: The girdle is the narrow section of the diamond separating the crown from the pavilion and functions as the diamond's setting edge as well as the blunt surface reducing the risk of damage from a blow to that edge. For round brilliant diamonds, facetted girdles with a thickness in the Thin to Slightly Thick range are generally preferred.
COLOR: On the GIA scale, diamond colors range from D (the highest and colorless) to Z (the lowest, light yellow). D, E, and F are considered colorless diamonds. G, H, I, and J are near colorless diamonds. Color grades H or I should be your target. Once mounted, these diamonds look just as good to the average person as the higher grades, but won't cost near as much. Look for a stone that is bright and has lots of fire since that is what will catch the eye. For round diamonds, at least I color is recommended and for fancy shapes, at least H color is recommended.
CLARITY: Clarity is the degree to which a stone is free from external marks called blemishes and internal features called inclusions. Inclusions normally have a greater impact on grade, value, beauty, and durability than do blemishes. Clarity ratings range from FI (flawless) to I1, I2, I3. The categories are FI (flawless), IF (internally flawless), VVS1 and VVS2 (Very, very small incusions), VS1 and VS2 (Very small inclusions), S1 and S2 (Small incusions) and I1, I2, and I3 (Imperfect eye visible inclusions). For diamonds with many facets (i.e., round brilliant), it is extremely difficult to see the difference between SI1 and higher grades even with a loupe or microscope. Therefore, target SI1 or VS2 as the best clarity values with outstanding beauty. For step cut shapes like emeralds and Asschers, at least VS2 clarity diamonds are recommended since these stones are so transparent and inclusions are easier to see with the eye.
CARATS: The weight of a diamonds is generally given in carats. Look for diamonds that have a diameter measurement that is at least as large as the average for that weight. In other words, don't pay for weight you can't see. The average diameter measurement for a .75 carat weight round diamond is 5.9 millimeters.