02.05.2022 Clarity, Color Quality & Fluorescence of Gemstones
When evaluating the quality of gemstones, the 4 Cs are central, i.e. Carat (weight), Color, Clarity and Cut. While we have already looked at what makes gemstones valuable in a more general article, here we would like to revisit factors related to the color of gemstones: Clarity, color quality, and the phenomenon of fluorescence. The evaluation of these aspects is made by gemological laboratories and recorded in the certificate of the gemstone.
Clarity in colored gemstones
A definite clarity scale has been specified for diamonds, but for colored gemstones, the expectation of clarity depends on the type of mineral and is judged less strictly. Inclusions affect clarity: These are foreign bodies that become trapped as the gemstones grow, whether they are other crystals being grown around, voids, liquids, or sediments. Inclusions can also be used as markers of origin, when certain types of inclusions indicate a mine or finding area.
Colored gemstones are classified into one of three categories to assess clarity, from Type I (eye clean – e.g., blue topaz, aquamarine) to Type II (smaller inclusions – e.g., ruby) to Type III (almost always included- e.g., emerald). The individual colored gemstone is then graded on a scale from just eye clean to opaque (“severely included”). This rating is shown on the gemstone’s certificate. It can be compared with the rating of other gemstones of the same type, i.e. the clarity of a blue sapphire with that of another blue sapphire. On the other hand, a green emerald with few inclusions can also be more valuable than an eye clean aquamarine.
Color quality of gemstones
Depending on the type of gemstone, certain color varieties are particularly sought after – such as “pigeon blood” rubies or royal blue sapphires. The color quality, i.e. how intense and rich the coloring is, has a major impact on the price, as does the even distribution of the color in the stone. Natural colored stones, whose color quality is unaffected by firing and other treatments, are considered more valuable than their treated counterparts. Great expert knowledge is required in determining the color and the value that comes with it.
Fluorescence in gemstones
When exposed to UV light, some types of gemstones show a different color – this phenomenon is called fluorescence because it was first observed in 1852 in the mineral fluoride. The physical process is as follows: the incidence of UV light raises electrons to a higher level. When they return to the ground state immediately afterwards, they emit energy in the form of light in the visible spectral range, hence the stone shows a different color. If the color effect lingers after the UV light exposure has stopped, it is called phosphorescence. Both fluorescence and phosphorescence are called luminescence. Other ways to trigger luminescence include electricity (electroluminescence) and X-rays (radioluminescence).
To date, about 200 minerals are known to fluoresce. If minerals contain iron, this prevents luminescence effects; on the other hand, uranium compounds, chromium, manganese and tungstates in the crystal lattice lead to fluorescence. Fluorescence occurs in many different colors; for example, yellow topaz shows orange fluorescence, opal shows blue fluorescence, and ruby shows red fluorescence. In diamonds, about one-third exhibit fluorescence, most of which show blue color under UV light, with yellow and white being rarer.
Fluorescence plays a role in the identification of gemstones in order to distinguish minerals of the same color. It can also be used to distinguish synthetic stones from natural ones.