Determining the true color of a gemstone can be tricky. The color we perceive is influenced by many factors, including the gem’s chemical composition, optical properties, cutting style, and lighting conditions. With so many variables, it’s easy to be fooled by appearances. However, by understanding what contributes to color, you can learn to properly judge gemstone color.
What makes gemstones colorful?
The source of color in most gemstones is trace impurities, known as color centers. These impurities get incorporated into the crystal structure as the gem forms within the Earth. Transition metal ions (like chromium, iron, or cobalt) are common color centers that produce vivid hues in gems like emerald, ruby, and sapphire. Some gems, like diamond, get their color from structural defects in the crystal lattice rather than impurities. The defects alter how light is absorbed and reflected to produce tints like yellow, brown, or even pink.
| Gemstone | Color Source |
|---|---|
| Ruby | Chromium impurities |
| Emerald | Chromium and/or vanadium impurities |
| Sapphire | Iron and titanium impurities |
| Diamond | Lattice defects |
The type and amount of impurities present affect the absorption of certain wavelengths of light. The wavelengths that aren’t absorbed are reflected back to our eyes, producing the color we see. For example, chromium absorption causes ruby’s rich red color. Emerald gets its signature green from absorption in the yellow to violet range.
Why does the same gemstone come in different colors?
Small variations in impurity content or structural defects can create big differences in color, even within the same gem type. Take sapphire for instance. Blue sapphire owes its color to iron and titanium impurities. But when more chromium is present, absorption shifts toward the red end of the spectrum, resulting in pink to orangey padparadscha sapphires. Completely colorless sapphires with very few impurities are also possible.
The same principles apply to other pleochroic gems like iolite, which can range from deep blue to yellowish gray depending on the viewing angle. Each color exhibits a different wavelength absorption. Pleochroism occurs when the crystal structure allows light to be absorbed differently along different optical directions within the gem.
Does the cutting style affect color?
Yes, the way a gemstone is cut can emphasize or mask certain color characteristics. Optimal cutting requires careful orientation of the rough crystal to showcase its best attributes. For strongly pleochroic gems like tanzanite, the cutter must align the gem to maximize the desired color. Cuts intended to emphasize color saturation or hue may differ from those focused on brilliance or sparkle.
Light performance is also impacted by cutting style. Well-cut facets allow light to reflect back to the eye before it can be absorbed. This amplifies the gem’s color intensity. But in some materials like aquamarine, deeper cuts promote the escape of lighter wavelengths, creating a more saturated blue.
Does color vary in different lighting?
It certainly does. Our perception of color is greatly influenced by the lighting conditions in which we view an object. Natural daylight tends to provide the truest color rendering. Incandescent bulbs emit warmer, yellower light that can subdue blue tones while enhancing yellows and oranges. Cool fluorescent lighting has the opposite effect, making warm hues appear muted. The direction of lighting also matters. Since gemstones sparkle and refract light in complex ways, color saturation can shift with the angle of illumination.
These factors make it essential to judge a gem’s color under consistent lighting. The industry standard is to use daylight-balanced fluorescent lamps in viewing booths or color grading rooms. Outside of controlled settings, it helps to compare stones side by side to accurately assess color differences.
How does color affect gem value?
In colored gemstones especially, color is king when determining value. Even small differences in hue, tone, or saturation can substantially impact price per carat. For example, deep red rubies with a vibrant, saturated glow command the highest prices. Stones that appear overly dark or exhibit brownish undertones are considered less desirable. Emeralds are most prized for their vivid green color in the bluish-green range.
Fancy color diamonds also draw premiums based on intensity, secondary hues, and uniform color distribution. A vividly saturated 1-carat yellow diamond could cost up to 10 times more than a pale yellow stone of similar size. Uniform color is ideal since large variations within a single stone reduce its value.
What are the best methods for assessing gemstone color?
Several techniques allow jewelers, gemologists, and appraisers to accurately determine a gemstone’s true color characteristics:
- Viewing under controlled, standardized lighting conditions – Illuminating the gem with daylight equivalent bulbs reveals the most accurate color rendition.
- Color grading by an expert – Professionals use master stone sets and viewing booths to pinpoint subtle differences in hue, tone, and saturation.
- Comparing to known samples – Direct side-by-side comparison with gems of documented color grades simplifies matching.
- Using color filtering lenses – Filters that isolate specific wavelengths help identify nuances.
- Examining with magnification – Strong magnification under fiber optic lighting reveals color zoning or anomalies.
- Advanced spectrometry – Instruments like spectrophotometers quantify color characteristics numerically through spectral analysis.
Conclusion
Color description in gemology is both an art and a science. Accurately determining a gemstone’s true color requires controlling lighting conditions, understanding optical phenomena like pleochroism, using comparative stones for reference, and a trained eye. By leveraging these best practices, jewelers and gemologists can feel confident assessing the color quality of any gemstone.
