1.
Evaluate the color. A bright, deeply colored gemstone can be cut almost any way that you desire. Brightly colored stones reflect light more easy and tend to mask interior inclusions. On the other hand, a light-colored gemstone will refract light more like a diamond and therefore should be cut in a way to maximize sparkle and brilliance.
2.
Examine the stone for flaws. Large interior flaws can muddy the look of any stone, but a dark gemstone can actually benefit from these flaws as they can make the interior light play appear brighter and more interesting. Some jewelers even made additional notches underneath dark gemstones to create additional sparkle. However, a lighter stone with major flaws will require skillful cutting to mask these flaws and their locations may determine what cut is best. For example, a light-colored stone with several large flaws will probably need facets on both the top and the bottom, as a cushion cut will provide.
3.
Determine how the stone will be set. They way that you plan to wear the stone should impact its cut. For example, if you intend to wear it every day in a bezel setting then you may need a cut that is easy to clean and that reflects light even from within a limiting bezel. On the other hand, if your gemstone is only to be worn on major occasions you may wish to indulge in a fancier cut or even a unconventional artistic one that would not be practical for everyday wear.
4.
Factor in the softness of the stone. A stone like an opal that is very soft will not hold a faceted cut. You need to consider whether you want a smooth, polished surface such as opals, turquoise and lapis lazuli tend to have, or a more faceted appearance like an emerald or ruby. Harder gemstones can also be polished smooth and cut without facets for an interesting look, but you should consult a licensed gem cutter before deviating too far from accepted styles in case you run a risk of ruining your gem.
5.
Consider the potential sparkle of the stone. A stone that will not sparkle because it is opaque should probably not get a faceted cut as it will simply be a waste. However, if a stone has good potential clarity then you should consider flattering facets to maximize its brilliance.
Always go with what makes you happy with your jewelry. In the end, you will be spending a fair amount of money on a custom cut piece, so you should select a cut that will make you pleased with your selection every time you wear it.