Alexandrite is a rare variety of chrysoberyl that looks green to bluish green in daylight and red to purplish red under incandescent light or candlelight. Chromium causes both the color and the change. With a hardness of 8.5 and good toughness, it wears well in rings, but fine stones with a strong color change are scarce and expensive, especially above one carat.
What alexandrite is
Chrysoberyl is a beryllium aluminum oxide. Most chrysoberyl is yellow, greenish yellow or brown, and some shows a cat's-eye effect. Alexandrite is the variety in which a small amount of chromium replaces aluminum and produces a distinct change of color between light sources.
Chrysoberyl crystallizes in the orthorhombic system. Crystals often grow as twins, and alexandrite commonly forms cyclic twins of three individuals that create a pseudo-hexagonal shape. Alexandrite is also strongly trichroic: viewed in different directions, the same crystal can show green, orange-yellow and purplish red. Cutters have to balance color change, face-up color and weight when they orient the rough.
How the color change works
Chromium in alexandrite absorbs yellow light strongly and leaves two windows of transmission, one in the blue-green and one in the red. The balance between them is close, so the color you see depends on the light. Daylight and fluorescent light are rich in blue and green, so the stone looks green. Incandescent bulbs and candle flames give off more red, so the stone looks red. The page on color in gemstones explains this and other causes of color.
Several other gems also change color, including some sapphires, garnets, spinels and fluorites. Trade custom reserves the name alexandrite for chrysoberyl. Other stones are described as "color-change sapphire" or "color-change garnet."
Discovery in the Ural Mountains
Alexandrite was first found in the 1830s in emerald mines near the Tokovaya River in Russia's Ural Mountains. It was named after the future Tsar Alexander II, and the traditional story places its discovery around his coming of age in 1834. Its red and green colors matched the colors of the Russian imperial military, which helped make the gem popular in Russia.
The Ural deposits produced the finest early alexandrites, with a bluish green daylight color and a raspberry red in incandescent light. Output declined over time, and Russian alexandrite became rare in the market by the 20th century. Old Russian stones remain a benchmark, though origin should be confirmed by a laboratory.
Other sources
- Sri Lanka has produced alexandrite from its gem gravels for a long time. Ceylon stones are often larger, with an olive or yellowish green daylight color and a brownish red change.
- Brazil became important after a find at Hematita in Minas Gerais in 1987. Some Hematita stones show a strong change from bluish green to purplish red. See gems of Minas Gerais.
- East Africa produces alexandrite in Tanzania, including the Lake Manyara area, as well as Zimbabwe and Madagascar.
- India, particularly Andhra Pradesh and Odisha, produces alexandrite of varied quality.
Alexandrite forms where beryllium-bearing pegmatite fluids meet chromium-rich rocks, a combination that also produces emerald. Many alexandrite deposits are in mica schists near ultramafic rocks, and much commercial material comes from alluvial or placer deposits. The page on the rock cycle and gem formation explains these settings.
How to judge color change
The strength of the color change is the main value factor. Gemologists describe it by percentage, comparing how much of the stone's face changes color and how completely it shifts.
- Look at the stone in daylight or under a daylight-balanced lamp, then under an incandescent bulb or candlelight. Compare the two colors side by side if you can.
- Note the colors. The trade prizes a vivid bluish green to green in daylight and a strong red to purplish red in incandescent light.
- Judge how much of the stone changes. A strong change across most of the face is better than a weak shift or one limited to a few facets.
- Check brightness in both lights. Dark stones can look nearly black in dim incandescent light.
- Consider clarity and cut. Eye-clean alexandrite is less common than eye-clean chrysoberyl, and fine stones are usually cut as mixed-cut ovals and cushions.
Stones with weak color change, or with brownish or grayish colors, cost far less than those with a dramatic shift. Size matters too: fine alexandrite over one carat is rare, and prices per carat rise steeply with size. The page on how colored gemstones are graded covers general grading factors.
Cat's-eye alexandrite
When alexandrite contains dense parallel needle-like inclusions and is cut as a cabochon, it can show both color change and a sharp band of light across the dome. Cat's-eye alexandrite is rare. Sri Lanka and Brazil are the usual sources. See gemstone optics for more on chatoyancy.
Synthetics and imitations
Synthetic alexandrite has been made in laboratories since the 1970s, by flux growth, by pulling crystals from a melt (the Czochralski method) and by other techniques. Lab-grown alexandrite has the same chemistry and properties as natural stones and can show a strong color change. Telling it apart requires careful study of inclusions, growth features and sometimes trace elements, so a lab report is wise for any costly stone.
Much "alexandrite" sold in inexpensive jewelry is not alexandrite at all. Synthetic corundum doped with vanadium, which changes from grayish blue to purple, has been sold under the name for decades. Synthetic color-change spinel and glass are also used. These imitations have a different refractive index and specific gravity from chrysoberyl, which a gemologist can test. Many people who believe they inherited an alexandrite ring from the mid-20th century own color-change synthetic sapphire.
Treatments
Alexandrite is not commonly treated. Some stones have surface-reaching fractures filled with oil or resin, which should be disclosed. Heat treatment is not used to improve the color change. The overview of gemstone treatments covers fracture filling in more depth.
Durability and care
At 8.5 on the Mohs hardness scale, alexandrite resists scratching well and has no easy cleavage, so it suits rings and everyday jewelry. Clean it with warm soapy water and a soft brush. Ultrasonic and steam cleaners are usually safe for untreated stones, but avoid them if the stone has fractures or fillings.
Alexandrite as a birthstone
Alexandrite is one of the June birthstones, alongside pearl and moonstone. It was added as a June stone in a revision of the American birthstone list in the mid-20th century, giving June buyers a transparent faceted option. In Russia, folklore held that alexandrite brought good fortune, and its red and green colors carried a link to the imperial army.
Frequently asked questions
Why does alexandrite change color?
Chromium in alexandrite absorbs yellow light and lets both green and red light through in close balance. Daylight, rich in blue-green, makes it look green, and incandescent light, rich in red, makes it look red.
Is alexandrite the same as chrysoberyl?
Alexandrite is a variety of chrysoberyl. All alexandrite is chrysoberyl, but only chrysoberyl colored by chromium with a distinct color change is called alexandrite.
How can I tell if my alexandrite is real?
Testing refractive index and specific gravity separates chrysoberyl from color-change synthetic corundum and spinel. Distinguishing natural from lab-grown alexandrite usually needs a gem lab.
Where is the best alexandrite found?
The original Ural Mountain stones set the standard, and some Brazilian stones from Hematita show a comparable strong change. Sri Lanka, East Africa and India also produce fine stones.