Cut Sometimes the shape and the cut of the stone is just as important as the the type of gem itself. A shape is the face-up outline form of the stone, whether it be round, oval, square, rectangular, pear, marquise or trillion. Given a stone of a certain shape, a gem cutter or lapidary can decide to cut that gem with different styles of facets, or no facets at all.
Color Not surprisingly, color is the key factor with colored gemstones. A common misperception in judging gems is that people assume the darker the color, the better the stone. This isn’t true: color can be too dark, as with some sapphires that look more black than blue. If a gem’s color is overly dark, it tends to be subdued and lifeless. A much better rule of thumb is the brighter, richer and more vivid the color, the better. Within each gemstone variety it is the clear, medium-tone, very intense and saturated basic color that is most preferred. Muted colors or colors between hues, which some might find very attractive, are usually less expensive. Always remember to look at the color in different kinds of light, since the light spectrum can affect gem color greatly.
Clarity A good cut, while it may not cost more, can add or subtract quite a lot of beauty to a stone. A well-cut, faceted gemstone reflects light evenly across its surface area when held face up. If the stone is too deep and narrow, areas will be dark. If it is too shallow and wide, parts of the stone will be washed out and lifeless.
Carat Weight Gemstones are generally sold by weight rather than size. Some gems are denser than others, so the same weight stone may be a different size! The carat weight will also affect the price. Large gemstones are usually rare, marking up the price per carat of the stone.