Alexandrite is a rare and fascinating gemstone that appears to change color depending on the light source. This optical phenomenon, known as the alexandrite effect, makes alexandrite appear greenish blue in daylight and purplish red under incandescent light. But why does alexandrite seem to switch between these two colors? The answer lies in the gem’s unique chemical composition and structure.
What Causes the Alexandrite Effect?
The alexandrite effect is attributed to the presence of both chromium and vanadium impurities in the gem’s crystal structure.
- Chromium is responsible for the emerald green color in daylight.
- Vanadium produces the deep raspberry red hue under incandescent light.
The amount of color change depends on the relative concentrations of these two elements. Stones with more chromium and less vanadium will appear greener, while a higher vanadium concentration results in a more pronounced reddish color.
The Science Behind Alexandrite’s Dual Colors
To understand what causes alexandrite’s color change, we need to examine how gemstones interact with light. The color we perceive is the result of selective absorption and transmission of certain wavelengths by the crystal lattice.
In the daylight, alexandrite absorbs violet and yellow light. This allows green and blue light to pass through, combining to produce the characteristic bluish-green color. Under incandescent light, the vanadium impurities absorb the green and blue wavelengths. Only red light is left to be reflected back to our eyes, resulting in the bold raspberry tone.
This phenomenon where a gemstone displays different colors under different light sources is called pleochroism. Alexandrite exhibits a strong dichroic effect, meaning it has two very distinct pleochroic colors.
Absorption Spectra of Alexandrite
The graph below illustrates the light absorption spectra of alexandrite under daylight and incandescent light conditions:
| Wavelength (nm) | Daylight Absorption | Incandescent Absorption |
|---|---|---|
| 400 (Violet) | High absorption | Low absorption |
| 500 (Green) | Low absorption | High absorption |
| 600 (Orange) | Moderate absorption | Low absorption |
| 700 (Red) | High absorption | Low absorption |
This clearly shows how alexandrite transmits different portions of the visible spectrum under different lighting.
Where Does Natural Alexandrite Come From?
Natural alexandrite is extremely rare, originally discovered in Russia’s Ural Mountains in the 1830s. It’s the official birthstone for the month of June. Here are some key facts about its sources:
- Russia remains the primary source, specifically the emerald mines in the Tokovaya River basin.
- Hematite replacement deposits in Minas Gerais, Brazil also produce fine alexandrite.
- Smaller quantities come from Tanzania, Sri Lanka, India, Madagascar, and Zimbabwe.
- In the US, small deposits have been found in North Carolina.
Due to its rarity, natural alexandrite commands very high prices, often over $10,000 per carat for fine gemstone quality.
Alexandrite Mining Locations
| Country | Key Mining Regions |
|---|---|
| Russia | Ural Mountains |
| Brazil | Minas Gerais |
| Sri Lanka | Ratanpuru |
| Zimbabwe | Masvingo |
| Tanzania | Tunduru |
| India | Kerala, Karnataka |
| USA | North Carolina |
Russia and Brazil account for most natural alexandrite on the market today.
Synthetic Alexandrite
Given the very high cost of natural alexandrite, synthetic versions have been produced in laboratories since the 1930s. These lab-grown stones exhibit the same chemical structure and pleochroic properties.
The most commonly used methods for synthetic alexandrite production include:
- Czochralski Process – Crystals are pulled from molten material.
- Flux Method – Crystals form in an aqueous solution.
- Kyropoulos Method – Crystals are grown in a small container from melt.
Although synthetic alexandrite has identical optics and even better color change, natural stones are still more valued by collectors and gemologists. Prices for synthetic material range from $300 to $500 per carat.
Comparing Natural and Synthetic Alexandrite
| Natural Alexandrite | Synthetic Alexandrite | |
|---|---|---|
| Origin | Mined from the earth | Lab-grown |
| Price per carat | $10,000+ | $300-$500 |
| Color change | Strong | Very strong |
| Inclusions | Common | Rare |
| Rarity | Very rare | Widely available |
While lacking the cachet and investment value of natural stones, lab alexandrite provides an affordable alternative for jewelry.
How to Tell Natural and Synthetic Alexandrite Apart
Distinguishing natural from synthetic alexandrite requires professional gemological testing, but here are a few identifying characteristics:
- Natural stones usually contain identifying inclusions like small crystals, feathers, and growth tubes visible under magnification.
- Lab-grown alexandrite will exhibit fewer inclusions and bubbles may indicate flux or hydrothermal growth.
- Natural alexandrite can show irregular color banding, while synthetic is typically more uniform.
- Advanced testing like spectroscopy and chemical analysis can conclusively identify origin and growth methods.
The most definite way to confirm natural alexandrite is through a certified gemological lab report from a respected organization like GIA or AGS. Reputable jewelers should provide such documentation for any high-value gemstone.
Unique Properties of Alexandrite
In addition to its famous color change, alexandrite has several other interesting properties:
- Hardness – With a Mohs hardness of 8.5, alexandrite is very durable for use in jewelry.
- Dispersion – High dispersion gives alexandrite exceptional fire and brilliance.
- Dense crystal structure – Refractive index of 1.74 to 1.75 contributes to its high light dispersion.
- Cat’s eye effect – Some stones exhibit an optical phenomenon called chatoyancy that produces a cat’s eye gleam.
The combination of color change, hardness, brilliance, and unusual optical effects makes alexandrite a truly one-of-a-kind gem.
Alexandrite Jewelry
The stunning color change of alexandrite makes it a unique choice for jewelry:
- Alexandrite engagement rings and wedding bands take advantage of the stone’s symbolism of joy and good fortune.
- Pendants and necklaces in white gold or platinum allow the alexandrite to stand out.
- Earrings and rings highlight the dramatic color change compared to the skin tone.
- Victorian and Art Nouveau styles complement alexandrite’s romantic yet mystical aura.
- Cat’s eye alexandrite works well in bolder, modern men’s rings.
Alexandrite looks beautiful in both white and yellow metals. Color change gems are often set in designs that allow light to pass through, like halo or kite settings. The June birthstone makes a perfect gift, symbolizing happiness and love.
Example Alexandrite Jewelry Designs
| Jewelry Type | Recommended Setting Styles | Metal Colors |
|---|---|---|
| Engagement ring | Solitaire, three-stone, halo | White gold, platinum |
| Pendant | Bale solitaire, sidestone halo | Yellow or white metal |
| Earrings | Stud, drop, floral motif | Rose or white gold |
| Cocktail ring | Bezel, three-prong setting | Yellow gold |
Alexandrite allows for very creative jewelry designs that show off its magical color change properties.
Caring for Alexandrite Jewelry
To keep alexandrite jewelry in excellent condition:
- Store pieces separately to prevent scratching other gems.
- Clean with warm water, mild soap, and a soft brush.
- Avoid steam cleaning and ultrasonic cleaners which can fracture alexandrite.
- Prevent sharp blows that can chip the stone.
- Take care when working with household chemicals.
- Avoid extreme temperature changes which can cause thermal shock.
Alexandrite’s durability makes jewelry care fairly straightforward. Regular cleaning and gentle handling will maintain its beauty for many years.
Conclusion
The mesmerizing color change of alexandrite has intrigued gem aficionados for almost two centuries. Its distinctive pleochroic properties are caused by trace elements in the gem’s crystalline structure selectively absorbing different wavelengths of light. Natural alexandrite remains an extremely rare and valuable gemstone, while synthetic versions offer an affordable substitute. The June birthstone carries symbolism of happiness and good fortune, making it a meaningful jewelry gift. Alexandrite’s dual green and purple hues will continue to captivate gem and jewelry lovers alike for generations to come.