Quick answer: Yes, the color of a gemstone can often help determine what type of gem it is. While color alone does not definitively identify a gem, it is one of the major factors gemologists use to categorize different gem varieties. Certain gems only occur in a limited range of colors, so the color can be a useful clue for identification.
The Role of Color in Gem Identification
The color of a gemstone is one of its most apparent and identifiable characteristics. Along with clarity, cut, and carat weight, color is considered one of the four C’s that determine a gem’s quality and value. While many factors are used to conclusively identify a gem, color is often the first clue that gemologists look for when categorizing a new specimen.
Each variety of gemstone has a typical color or range of colors that occurs naturally. For example, natural diamonds are usually colorless or occur in various tints of yellow, brown, or gray. Rubies are defined as being red. Emeralds are green. Sapphires can occur in all colors except red, but are most typically blue.
If a gem does not display the expected range of colors for that variety, it may be an indicator that the specimen is not what it appears to be. Color outside the usual range could mean the gem is a different variety altogether, or that it has been treated or enhanced in some way. Understanding the typical color range for each type of gem helps gemologists identify stones and determine if their color is natural or modified.
Color Origins in Gemstones
The color of any gemstone is the direct result of its chemical composition and structure. The minerals that make up the gem contain elements that interact with light to selectively absorb, reflect, and transmit different wavelengths of light. The wavelengths that are transmitted or reflected determine what color our eyes perceive. For example, ruby gets its red color from trace amounts of chromium. Emeralds contain small amounts of chromium and/or vanadium that produce its characteristic green.
The crystal structure of the mineral also affects how the elemental components interact with light. Chemical impurities, radiation exposure, structural defects, and other factors can all influence the final color that is visible. While the exact chemical and structural causes of color are complex, gemologists have identified the typical color-causing agents in most gem types.
Color Zoning
While some gems occur in uniform solid colors, many exhibit multiple colors in bands, patches, or other patterns. These multi-color gems are referred to as color zoning. Color zoning occurs when the gem’s chemical composition or crystal structure varies slightly in different areas. This can form stripes, blotches, or gradual color gradations within a single stone.
Some examples include watermelon tourmaline, which has green exteriors and a pink or red center, and bi-color or parti-colored sapphires displaying two distinct color zones. Alexandrite is also famous for exhibiting one color in daylight and a different color under incandescent light. Color zoning can be a naturally occurring phenomenon or produced artificially during the cutting process.
Effects on Value
As a major factor in gemstone appeal, color also significantly influences a gem’s value and desirability on the market. Stones with colors that are pure, intense, and uniform typically command the highest prices. Unique or rare colors also attract interest from collectors and buyers.
On the other hand, overly light, weak, or irregular color detracts from value. While the same mineral, a gem with less desirable color will be priced lower than a top-quality example. For particularly valuable varieties like emerald and ruby, stones are graded based on color saturation and separated into commercial classes. Maximizing perceived color is one of the primary goals when facet cutting gems to improve their marketability.
Typical Gemstone Colors
Here is a table summarizing the colors associated with some major gemstone varieties:
| Gemstone | Typical Colors |
|---|---|
| Diamond | Colorless, yellow, brown, pink, blue, green |
| Ruby | Red |
| Sapphire | Blue, yellow, pink, purple, green, orange, black, colorless |
| Emerald | Green |
| Amethyst | Purple, violet |
| Citrine | Yellow, gold, orange |
| Peridot | Green |
| Aquamarine | Blue, green |
| Garnet | Red, pink, green, orange, purple, colorless |
| Opal | White, black, colorless, multi-color |
| Topaz | Blue, yellow, pink, colorless |
| Tourmaline | Pink, green, blue, red, yellow, black, colorless |
This table demonstrates the strong color associations that help distinguish gem varieties, even though there can be overlap between some types. A blue sapphire and green emerald are obvious examples where color is the definitive factor in identification. But even colorless diamonds, garnets, and topaz get their identity primarily from being in that expected color range.
Color as an Identification Tool
When attempting to identify an unknown gem sample, a gemologist will first look at color as an early indication of what the stone could be. They will use the color as a screening tool to narrow down the possibilities. From there, further testing is done to confirm the variety based on other optical, chemical, and physical properties. But color gives the initial clues that point the tester in the right direction for identification.
For the gemologist, recognising if the color is typical or unusual for that variety is also important. An odd color may be a red flag that the stone is not natural or has been treated in some way. Color testing under different light sources is also used to detect any issues. So color acts as a quick preliminary identifier as well as checking for any color anomalies.
Color Grading
Once the variety of gem has been established, the color is then graded on a scale to determine quality. Color grading sets guidelines for hue, tone, and saturation that determine the perceived attractiveness of the color. Stones at the top end of the scale have colors rated as more desirable. Those near the bottom are considered low quality for that variety in terms of color.
For example, diamond color grading runs from D (completely colorless) to Z (light yellow/brown). Ruby color quality extends from vivid red to light red. Sapphire grading scales from blue to green and also dark to light for each hue. Emerald grading considers tone and saturation of green. Each gem type has its own color grading system tailored to that mineral. Grading reports will include an assessment of color quality that helps determine market value.
Treatments to Alter Color
Since color is so central to gem value, treatments have been developed to improve or change the color of certain stones. These may imitate a more desirable hue or intensify pale color to increase quality. Treatments include heat, radiation, coating, dyeing, and many other methods. Some are permanent while others are unstable.
Disclosure about any color enhancement is critical for ethical trading. Savvy buyers should be aware of what gem colors can be artificially modified and look for certification that discloses any treatments. Improper color treatments without disclosure are one way consumers can be misled about gem quality.
Special Color Phenomena
Some rare gems have special color properties that make them novel and valuable. Changeable color in different lighting, like alexandrite or color-change sapphire, is a highly desirable trait. Opals display flashes of spectral color called opalescence. Other gems, like labradorite, exhibit iridescence and schiller effects through optical diffraction. And some phenomenal gemstones like star rubies and cat’s eye chrysoberyl display optical effects related to their unusual crystal structure. Stones exhibiting these special color behaviours are prized by collectors and command high prices.
Conclusion
In summary, color is one of the most useful initial characteristics for identifying unknown gems. While color alone cannot definitively diagnose gem type, it provides important clues that can aid identification when combined with other testing. Each variety of gem displays a typical range of colors that is an integral part of its description. The attractiveness and value of a gemstone depends heavily on its color quality. Maximizing color is a major goal in cutting gem rough into finished stones. Both natural color variations and artificial treatments influence gem values. For these reasons, color is a fundamental consideration in gemology and the gemstone marketplace. An understanding of typical gem colors and causes of color provides insight into gem identification, enhancing appreciation of these colorful natural treasures.
