The market has established different milestone value for different weights. Cut Quality of the cut may be the most important factor that determines a diamond's value. A poorly cut diamond is often valued at what is would weigh if it were recut in the proper proportions to achieve the maximum brilliance. Therefore, a poorly cut one-cart stone may really have the value of a .75 carat stone. Also make sure that the diamond isn't too heavy or too shallow.
Shape is also important.
Get the one that you like. A round diamond is considered to be the standard of the industry. Contrary to popular opinion, many cuts such as emerald, oval, or hearts are less valuable. Other cuts such as marquis may bring a premium price due to the current demand in the market today. You can't see the true color of the diamond if it is moundted in a ring.