Emeralds have long been prized for their signature green color. However, some emeralds can display different hues, ranging from blue to yellow. While pure emerald is always green, impurities in the crystal structure can modify the color. Additionally, some treatments applied to improve clarity affect the color as well.
What gives emeralds their green color?
The vivid green color of emeralds is due to the presence of chromium and/or vanadium. These trace elements get substituted for some aluminum atoms in emerald’s crystal structure during its formation. The chromium and vanadium absorption bands interact with the crystal field to product the characteristic green electromagnetic spectrum for emerald.
Emerald is a type of beryl, with the chemical formula Be3Al2(SiO3)6. The color-causing chromium and vanadium substitute for some of the aluminum atoms in the lattice. The resulting color depends on the oxidation state of the chromium ions. Cr3+ ions produce a strong green color, while Cr2+ ions contribute more bluish hues.
How do impurities modify emerald color?
While chromium and vanadium cause the green color, other impurities can modify the resulting hue. The presence of iron ions tends to darken the green. Iron can also combine with chromium to produce blue-green colors.
When vanadium substitutes for aluminum, it often occurs alongside iron. The color intensity depends on the ratio of vanadium to iron. Higher vanadium contributes yellow hues, while higher iron adds blue. The combination of the two produces green.
Rare emerald colors
While green is by far the most common, emeralds can occur in other colors when the chemical composition is different.
Blue emeralds
Emeralds owe their blue hues to the presence of iron and coupled substitution of iron and chromium in the place of aluminum. The more iron substitutes, the bluer the color. However, the amount of chromium still needs to be significant to produce a discernible green-blue rather than simply pale blue.
Colombian blue emeralds are the most famous. They occur in the Muzo mine. Trace amounts of nickel also seem to enhance the blue tones. Habitually blue emeralds are dark and low in saturation.
Yellow emeralds
Yellow emeralds receive their color from vanadium or other transition metals substituting for aluminum. While pure chromium produces green, adding more vanadium takes the color toward yellow-green. When vanadium content gets high enough, it can produce rare yellow emerald crystals.
Most famous is the Canar?? Emerald, discovered in Brazil in 1987. Weighing 1,383 carats, it possesses a deep golden yellow color.
Pink emeralds
While pure emeralds are only colored by chromium, vanadium, and iron impurities, other trace elements can provide unusual hues. Manganese and titanium are often credited with causing pink emeralds. These trace elements modify the crystal field to allow new energy transitions that give rise to pink.
Pink emeralds were first discovered in the late 1980s in Russia. More recently, others have been found in Africa. However, they remain exceptionally rare.
Red emeralds
Red emeralds receive their color from chromium, similar to blue and green emeralds. However, they also possess additional impurities that modify the crystal field to produce longer-wavelength red light.
The first red emerald was discovered in North Carolina in 1998. Since then, others have been found in Asia. Like pink emeralds, red emeralds are exceptionally rare.
Effects of treatments on emerald color
Since emeralds often possess inclusions that reduce transparency, they frequently undergo treatments to improve clarity. However, these can also impact color.
Oils and resins
Emeralds are commonly infused with clear oils and resins to fill cracks and fissures. This improves clarity but can also slightly darken the emerald green color, especially if higher refractive index oils are used.
Dyeing
Unscrupulous emerald traders sometimes dye cracks in gemstones to obscure clarity issues. Organic dyes used include ink, plant extracts, and green oil. These can turn lighter emeralds a more intense green but have a destructive effect on value.
Irradiation
Exposure to radiation alters trace impurities in the crystal structure, causing color changes. Irradiated emeralds undergo a process called the photochrome effect. Their color shifts upon exposure to long-wave ultraviolet light. Colored oil or resin treatments often follow irradiation.
Diffusion
Diffusion treatments introduce new chemical elements into a shallow surface layer on the emerald crystal. Heat drives the diffusion process. Elements like chromium and iron can enhance green color, while cobalt produces blue. However, excessive diffusion performs can leave unnatural color zoning.
Identifying emerald color treatments
Color enhancement treatments make it essential to test emeralds for any modifications. Common methods used include:
- Ultraviolet testing – Observing color changes under UV light can reveal irradiation and dyeing.
- Microscopic inspection – Fissure fillings are visible under high magnification.
- Chemical testing – Acid tests detect surface coatings and diffusion treatments.
- Spectroscopic analysis – Labs can detect any elemental changes to the crystal composition.
Reputable gemological labs certify all enhancement treatments performed on emeralds submitted for analysis.
Conclusion
While green is the classic, pure emerald hue, other colors occur when trace impurities modify the crystal structure. Blue, yellow, pink, and red emeralds exist but are exceptionally rare. Treatments like dyeing and diffusion can also manipulate the color. Careful testing by gem labs identifies any color enhancement in emeralds.
