When assessing the quality of gemstones, the 4 Cs are of paramount importance: Carat (weight), Color, Clarity, and Cut. In this article, we will focus on the specific factors related to the colour of gemstones: Clarity, colour quality, and the intriguing phenomenon of fluorescence. Gemological laboratories evaluate these aspects and record their findings in the gemstone's certificate.
Clarity in Colored Gemstones
While diamonds have a well-defined clarity scale, the expectations for clarity in colored gemstones vary depending on the mineral type and are judged less strictly. Clarity is influenced by inclusions, which are foreign materials trapped within gemstones during their formation. These inclusions can range from other crystals to voids, liquids, or sediments. In some cases, specific types of inclusions can serve as markers of a gemstone's origin, indicating a particular mine or finding area.
Colored gemstones are categorized into three groups for clarity assessment: Type I (eye clean, e.g., blue topaz and aquamarine), Type II (smaller inclusions, e.g., ruby), and Type III (almost always included, e.g., emerald). Each individual colored gemstone is then graded on a scale from eye clean to opaque (severely included), with this rating noted on the gemstone's certificate. This clarity rating can be compared with other gemstones of the same type. Consequently, a green emerald with minimal inclusions might be more valuable than an eye-clean aquamarine.
Color Quality of Gemstones
The expected clarity of coloured gemstones depends on the specific mineral type. Certain colour varieties are highly sought after, such as "pigeon blood" rubies or royal blue sapphires. The quality of the colour, which refers to its intensity and richness, significantly impacts the gemstone's price, as does the even distribution of colour within the stone. Natural coloured stones, unaffected by treatments like heat, are considered more valuable than treated counterparts. Assessing the colour and its associated value requires extensive expertise.
Fluorescence in Gemstones
Some gemstones exhibit a different color when exposed to UV light, a phenomenon known as fluorescence. This term originated from its initial observation in the mineral fluoride in 1852. The process involves UV light causing electrons to move to a higher energy level, and when they return to their ground state, they emit visible light, altering the stone's colour. If this colour change persists after UV exposure ends, it is termed phosphorescence. Both fluorescence and phosphorescence are collectively referred to as luminescence. Other methods for inducing luminescence include electricity (electroluminescence) and X-rays (radio luminescence).
Approximately 200 minerals are known to fluoresce, with luminescence typically influenced by the presence of uranium compounds, chromium, manganese, and tungstate within the crystal lattice. Different gemstones exhibit various fluorescence colours. For example, yellow topaz displays orange fluorescence, opal shows blue fluorescence, and ruby exhibits red fluorescence. In diamonds, about one-third exhibit fluorescence, primarily in shades of blue under UV light, although yellow and white fluorescence is less common.
Fluorescence serves a critical role in gemstone identification, enabling the differentiation of minerals with similar colours. It is also useful for distinguishing synthetic gemstones from natural ones.