This guide will help you to understand what qualities and features to look for when you are purchasing a gemstone and recognize several essential characteristics of quality gemstones.
What to look for
A good cut showcases the gemstone's colour, diminishes its inclusions, and exhibits good overall symmetry and proportion. Because gemstone colour can vary, there are no hard geometrical standards when it comes to maximizing brilliance or colour. Gemstones, especially rarer ones, are sometimes cut for size without regard for their colour.
In a gemstone with more saturated colour, the best cut may be more shallow than average, permitting more light to penetrate the gemstone, while in a less saturated gem, the colour may benefit from a deeper cut.
Look at the gemstone in the setting and ensure that all the facets are symmetrical. An asymmetrically-cut crown indicates a gemstone of low-quality. In all cases, a well-cut gemstone is symmetrical and reflects light evenly across the surface, and the polish is smooth, without any nicks or scratches.
Like diamonds, fine quality colour gems usually have a table, crown, girdle, pavilion, and culet. Iridescent opals are one exception, and most often have a rounded cabochon cut.
The carat weight of a gemstone does not necessarily allow you to accurately envision the size of the gemstone. Different gemstones have different densities (mass per unit volume), so two gems that appear to be the same size may actually have very different weights. For example, a ruby is more dense dense than a diamond, so a 1-carat ruby will look smaller than a 1-carat diamond.
Always request the dimensions of a gemstone to ensure that the majority of the gemstone weight will be visible when set in the setting.
The beautiful colour of a gemstone is its most defining characteristic, and many jewellers consider it to be the most important evaluation criterion. When deciding upon gemstone colour, examine hue, tone, and saturation.
The most valuable gemstones are those that exhibit a pure colour and only "slight" hues of other colours in addition to their primary colour.
Tone represents the depth of colour, ranging from colourless to black. Gemstone tone is described as "light," "medium-light," "medium," "medium-dark," and "dark."
Saturation, or colour purity, refers to the degree to which the gem is free from brown or gray hues. The most desirable gemstones, which show little gray or brown, are often described as having "vivid" or "strong" colour saturation.
Because gemstones form under unique circumstances, each individual gemstone is comprised of a combination of trace minerals, which create a unique set of identifying marks or inclusions. Inclusions will not necessarily detract from the beauty or desirability of a coloured gemstone.
What to look for
When considering a coloured gemstone's clarity, you should measure your expectations against the standard for that variety of gemstone. Some varieties of coloured gemstones, such as aquamarine, blue topaz, and citrine, have naturally fewer inclusions while other gemstones, such as emerald and ruby, tend to have a higher rate of acceptable inclusions.
Clarity is an important factor in comparing quality coloured gemstones. Even opaque opals can have milky inclusions that will effect their iridescence and colour play. In general, the best values are available in gemstones that are moderately included. Exceptional gemstones with few or no inclusions are available, but they can command extravagant prices.
Unpolished gemstones are typically very rough. Looking at gemstones straight from the mine, they might be mistaken for pebbles or gravel.