Aside from the right overall proportions and a shape that suits its owner, a diamond's value and intrinsic beauty are governed by additional aspects of the workmanship of the diamond cutter.
When looking at diamonds under a microscope, compare the form of each facet with the ideal forms presented here. For example, a round diamond has exactly 58 facets -- 33 above the girdle, and 25 below.
When looking at actual stones, refer to the diagram above and try to identify and rate each of the facets. Extra, absent, or deformed facets decrease the value of the stone. Refer to the diagrams in the other sections of this tutorial for top views of different diamond shapes.
The girdle of a diamond is the middle, or fattest part, of the diamond, and is measured from "extremely thin" to "extremely thick":
Thin girdles are prone to chipping. Extremely thick girdles hide a lot of weight, so you end up paying for a heavier diamond that doesn't look that big. The ideal is an even, medium girdle; this applies to all shapes EXCEPT for the tip of the pear shape, in which a thick girdle is preferred to make it less prone to damage. When examining the girdle, make sure that it is well-faceted and polished, since some diamond cutters will omit this step to save time and to avoid removing weight from the stone.
The culet is the bottom part of the diamond, where the pavilion comes together in a point (refer to the diagram shown above, under "facets"). Culets are rated as "none", "small", "medium", and "large." The ideal is "none," but if an otherwise perfect stone has a medium or large culet, it may still be a worthwhile purchase.
Check the following aspects of the stone to make sure it is completely symmetrical:
There's no hard and fast rule to judge the finish of a stone. However, understand that the more a diamond cutter polishes a stone, the more carat weight it loses (and therefore its value), so the cutter's goal will be to polish just enough to satisfy the potential customer. Polishing will also remove any surface flaws relating to clarity, so that potentially an IF diamond could be turned into an FL simply by polishing it.