In colour science, there are three dimensions for colour: hue, saturation and tone (light or dark). When evaluating the colour of coloured gemstones, however, there is another aspect to consider: uniformity of colour.
The optimal colour range (combination of hue, saturation and tone) is different for each gemstone, and for many gemstones the availability of the optimal colour can be low. Pale colours usually have a lower value ( but there are also exceptions - like pale colour untreated Topaz ( see Fig.1 ) or "mint" pale colour Tourmaline ).
Any uneven distribution of colour within a gemstone is called colour zoning. To better observe colour zoning, turn the gem upside down on a white piece of paper and look for uneven coloration.
Fig.1 ( untreated pale blue Topaz, Namibia )
Clarity is an important value factor because gems are cut to sparkle and show off their colour in interesting ways. If there are flaws that interrupt that sparkle, the gem is less interesting.
It`s important to know, there are also exceptions:
There are some inclusions that actually help the value of specific gems. For example: Sunstone is also aided by light amounts of schiller (extremely tiny copper inclusions that create a cloud-like appearance, see Fig.2), which in the right locations can add value. Also nice rutile inclusions can add the value ( see Fig.3, Myanmar Topaz with golden Rutile inclusions ).
Fig.2 ( Oregon Cooper Sunstone, USA )
Fig.3 ( Gold rutile Topaz, Myanmar (Burma) )
Size, which includes the weight and face-up diameter of each type of gem material, is also directly related to value (in connection with a good cut - the cut quality must never be sacrificed to the weight).
When center pavilion facets are cut too shallow for that gem material, light passes through so that we see what is behind the gem. This is called windowing. These cutting styles are rarely attractive.
Today custom cut or modified "conventional" cuts are very popular and often add value. ( see Fig.4 )
Fig.4 ( Custom cut Topaz, Namibia (UNTREATED / NATURAL LIGHT BLUE) )
As a consumer, you will regularly encounter in the marketplace gems that have been treated to change their appearance. A topic that often comes up is whether a particular gemstone is or isn’t treated. Natural gem crystals are transformed from their rough crystallographic form into the shapes, outlines, and degrees of polish in the gemstones that we appreciate and wear in jewelry. These steps are and have always been the routine procedures used for manufacturing gemstones. Beyond traditional cutting and polishing, however, gems can often be treated in ways meant to alter their colour or clarity.
The most important factors for a higher value are that the gem was mined from the earth (not synthetic) and is best left untreated. Light heating can be described as a "natural process", which is common and accepted for many stones today. Any other treatments (such as diffusion treatment, lead glass treatment, oil treatment (Emeralds) etc.) are usually worth reducing.
Tourmaline with natural inclusions under Microscope
"My passion belongs to the gems mined and untreated from the earth"