Like diamonds, gemstone quality and value are evaluated according to the four C's: color, clarity, cut and carat weight. One other factor that is less scientific - perception - is also important. When buying a gemstone, adhere to the Four C's, being especially mindful of cut and color.
Color is the key factor. Brighter, richer and more vivid color is better. Do not assume when judging gems that the darker the color, the better the stone. Color can be too dark making the gem look subdued and lifeless, as with sapphires. They look more black than blue when too dark.
Within each gemstone variety look for a clear, medium-tone; very intense and saturated basic color is most preferred. Less expensive gemstones have muted colors or colors between hues. Since light spectrum can affect color greatly, look at the color in different kinds of light.
Look for rich color, translucency and brilliance when buying a ruby, sapphire, emerald or any other colored gemstone. Look for vibrant color rather than deeper color. Transparency and translucency are essential in colored gemstones. Most gemstones have some inclusions because they are formed in nature. Some are invisible to the naked eye. Emeralds, tourmalines, peridots and rubellites have more inclusions than other stones. If their color is strong inclusions are not always a negative factor.
Gemstone clarity is the next most important factor affecting value. Clear, transparent gemstones with no visible flaws are the most valued. Gemstones such as emeralds and red tourmaline are very rare without inclusions making them higher in value. Other gems are valued for their inclusions. It is the tiny inclusions reflecting back the light that puts the eye in cat's-eye chrysoberyl. Inclusions reflecting back the light create the star in star sapphire and ruby. Where a gemstone is mined can be determined by the birthmark created by the inclusions.
A good cut can add or subtract a lot of beauty. When held face up, a well-cut gemstone reflects light back evenly across the surface area. There will be dark areas if the stone is cut too deep and narrow. Parts of the stone will be washed out and lifeless if it is too shallow and wide. A gemstone should have an even brilliance with no parts looking shadowed or dark. Improperly cut diamonds, especially pear, oval and marquise shapes , can create an undesired "bow-tie" shadow effect when looking at it. Facets should be evenly and symmetrically sized and well-polished. The outline of the stone should be even and well-defined.
Gemstones are not sold by size. They are sold by carat weight. The same weight gemstones may be a different size because they are denser. Large gemstones are usually more rare, making the price per carat higher.
Let your eye be the judge (perception) when buying gemstone jewellery. Harmony and balance are important. The stone and ring shank should be in proportion in a gemstone ring. A very large stone balanced on a thin ring is unbalanced. A very thick band and heavy setting for a small stone is not desirable.
Different setting techniques and several different sizes and shapes of stones are used to achieve a unique look in the best-designed pieces of jewellery. For example, an engagement ring may have a round, brilliant center stone held in place with six prongs. This is known as the Tiffany setting. For variety, the center stone might be bordered on two sides by rows of diamond baguettes in a channel setting.
When a piece of gemstone jewellery is set using a prong setting, the prongs should be balanced evenly around the stone and should hold the stone securely.
A bezel setting is a sort of gold picture frame of metal around the stone. The bezel should be straight an uniform and the stone should not move within the setting.
In channel or invisible settings, many stones are set tightly together with no metal showing. The stones should be all the exact size, shape and color. No one stone should be more prominent than the other with facets all lined up.
In a pave setting tiny stones are set into a gold surface. The gold is pushed up around each stone using a hand-held tool. Nothing but the sparkling surface of the diamonds should be visible. It is not good workmanship if the surface is uneven, or one stone is higher or lower than the ones around it. Usually, white diamonds are pave set in platinum or white gold, but also set in yellow or pink gold.
Turn over a piece of jewellery to judge the quality. The back should be as beautiful as the front. It should be polished and beveled nicely, not rough or unfinished looking. In a piece of jewellery with several stones set closely together, the metal should be cut away in square or honeycomb patterns at the back. This is called gallery work, which allows the maximum amount of light to shine through.