Alexandrite Colour Change: Why It Turns Green to Red, and What to Check

What is alexandrite?

Alexandrite is a rare colour-change variety of the mineral chrysoberyl, a beryllium aluminium oxide with the formula BeAl₂O₄. Ordinary chrysoberyl is usually yellow, greenish yellow or brown. Alexandrite is the version that carries a trace of chromium, and that tiny impurity is what makes it look green in daylight and red under warm lamplight. It is one of the most unusual stones in our guide to coloured gemstones, and many people first meet it as one of the three June birthstones, alongside pearl and moonstone.

The story usually told is that alexandrite was found in the emerald mines of Russia's Ural Mountains in the 1830s and named after the future Tsar Alexander II. Its two colours, red and green, echoed the Russian imperial military colours, which helped make it fashionable at court. The Ural deposits produced little after the 1800s, and most alexandrite sold today comes from Sri Lanka, Brazil, India and East and southern Africa.

In numbers, alexandrite is 8.5 on the Mohs scale (a 1–10 scale of scratch resistance), has a refractive index of about 1.746–1.755 (a measure of how strongly it bends light) and a specific gravity of about 3.73, so it feels slightly heavy for its size. Any unfamiliar term here is explained in our plain-English jewellery glossary.

• Chrysoberyl: the parent mineral, usually yellow to greenish yellow, with no colour change.
• Cat's-eye chrysoberyl: chrysoberyl with fine parallel needles that create a sharp line of light across a domed cabochon, an effect called chatoyancy.
• Alexandrite: chromium-bearing chrysoberyl with a distinct change from green to red.
• Cat's-eye alexandrite: very rare stones that show both the colour change and the cat's-eye line.

What causes the alexandrite colour change?

Every gem gets its colour by absorbing some wavelengths of light and letting the rest pass to your eye. The chromium in alexandrite absorbs strongly in the yellow part of the spectrum, centred around 580 nanometres, and also takes out some violet. What gets through is two windows: one in the blue-green and one in the red. The important part is that those two windows are almost evenly balanced, so the colour you see depends on which one the light source feeds more strongly.

Daylight and cool white light contain plenty of blue and green, so the green window wins and the stone looks green to bluish green. A candle, a traditional incandescent bulb or a warm-white LED gives out far more red than blue, so the red window wins and the stone looks red, purplish red or raspberry. Your eye also adjusts to the light around you, which pushes the effect further. The phenomenon is called the alexandrite effect, and gemmologists use the same name when other stones do it.

Chromium is the same element that makes ruby red and emerald green. In ruby the balance tips firmly towards red; in emerald, firmly towards green. Alexandrite sits almost exactly between them, which is why it is often described as 'an emerald by day, a ruby by night'. That is a helpful picture, but in real stones the daylight colour is more often teal or bluish green and the evening colour a purplish red. If you want the vocabulary for describing those colours precisely, see our guide to how hue, tone and saturation describe a colour.

Colour change is not the same as pleochroism, which is when a stone shows different colours from different viewing directions under the same light. Alexandrite has both. It is strongly trichroic, showing green, orange-yellow and purplish red along its three crystal directions. A skilled cutter orients the rough so the face-up colour and change are as strong as possible, often at the cost of weight, which is one reason fine alexandrites tend to be small.

Does alexandrite change colour under LED lights?

Usually yes, but it depends on the LED. The useful number is colour temperature, measured in kelvin (K). Low numbers are warm and reddish; high numbers are cool and bluish. Here is what a typical good alexandrite does under common light sources, which answers the classic question of alexandrite in daylight vs incandescent light.

LEDs have uneven, spiky spectra, so two bulbs with the same kelvin rating can show an alexandrite slightly differently. Mixed lighting, such as a warm lamp in a room with daylight coming in, often gives a muddled colour with flashes of both. Many Australian homes now use warm-white LEDs, which suit the red side well, while shops and offices often use neutral or cool lighting that shows the green side.

Photos and videos can mislead. A phone or camera automatically corrects white balance, which can exaggerate the change or flatten it. When you compare stones online, look for videos filmed under a stated light source, and ideally both colours shown side by side with a white reference card in frame. The same caution applies to colour-change sapphire, which shifts from blue to violet.

How is alexandrite colour change graded?

There is no single, universal grading scale for alexandrite, the way diamonds have D-to-Z colour. Laboratories such as GIA describe the colours seen under daylight-equivalent and incandescent light on the report, and some labs add a comment on the strength of the change. In the trade, dealers often describe strength as a percentage, such as an '80% change', but that is a visual estimate, not a measurement, and two people can disagree by a wide margin. Coloured diamonds face a similar challenge, as our explanation of how fancy colour diamonds are graded shows.

What gemmologists actually look at:

• Strength of change: how completely the stone switches between its two colours. This drives value more than anything else.
• Purity of the two colours: a bluish green and a purplish red are preferred over yellowish green and brownish red.
• Tone and saturation: medium to medium-dark tones with good saturation. Very dark stones can look almost black in dim light; very light stones show little change.
• Clarity: eye-clean stones are rare above about 1 carat, so small inclusions are accepted more readily than in sapphire.
• Cut and size: good orientation and even brightness, and weight, because strong-change stones over 1 carat are scarce.

There is genuine debate about where colour-change chrysoberyl ends and alexandrite begins. Most labs want to see a distinct change towards red before they use the name. If a stone is described as alexandrite but the report only mentions a slight shift, ask about it. Our page on the three parts of gemstone colour is a good primer before you compare reports.

Where does alexandrite come from, and is there Australian alexandrite?

Australia is a major source of sapphire and opal, and until 2020 it supplied most of the world's pink diamonds from Argyle (our guide to heart shaped and pink diamonds covers those). It is not, however, a recognised commercial source of alexandrite. Chrysoberyl has been recorded in some Australian localities, but we are not aware of Australian gem alexandrite reaching the market in any quantity. In practice, nearly every alexandrite sold in Australia has come from overseas.

A lab can sometimes give an opinion on origin, using inclusions and trace-element chemistry. Origin can affect price, with Russian and Brazilian stones attracting premiums, but origin alone does not make a stone look better. Judge the change you can see first, then the report.

How can you tell if alexandrite is real, lab-created or an imitation?

There are three groups of stones sold with the alexandrite name, and only two of them are actually alexandrite.

• Natural alexandrite: mined chrysoberyl with a natural colour change. Rare, especially in sizes over 1 carat.
• Lab-created alexandrite: also called synthetic alexandrite. It has the same chemistry and crystal structure as natural, and shows a real colour change, often strong and clean. It has been produced commercially since the 1970s by flux growth, by the Czochralski method (pulling a crystal from a melt) and by floating-zone growth. It must always be sold as lab-created or synthetic.
• Imitations: materials that copy the effect but are not chrysoberyl. The most common is vanadium-doped synthetic corundum, which shifts from greyish blue to purple or mauve and fills many old souvenir and birthstone rings. Synthetic colour-change spinel and glass are also used.

Because lab-created alexandrite has the same refractive index and specific gravity as natural, a refractometer alone will not separate them. A gemmologist relies on inclusions under the microscope and, for difficult stones, a lab uses spectroscopy and trace-element analysis. For any natural alexandrite of real value, ask for an independent report from a recognised lab. Lab-created alexandrite is a genuine choice in its own right: it is real chrysoberyl, just grown rather than mined, and costs a small fraction of natural.

If you love the effect but a fine natural alexandrite is out of reach, natural colour-change garnet and natural colour-change sapphire from Sri Lanka and East Africa are worth comparing. Both are mined stones, and sapphire is harder again at 9 on the Mohs scale.

Is alexandrite good for an engagement ring?

Yes. Alexandrite is one of the more practical coloured stones for daily wear. At 8.5 on the Mohs scale it resists the everyday dust and grit (mostly quartz, at 7) that slowly dulls softer gems, and its toughness, its resistance to chipping and breaking, is rated excellent. It has no easy cleavage plane, unlike diamond or topaz, so a knock is less likely to split it. Here is how it compares for rings:

That puts alexandrite well ahead of tourmaline, a softer but colourful ring stone, and in the same practical bracket as sapphire. Because most natural alexandrites are under 1 carat, popular designs include a small centre stone with a diamond halo, a three-stone ring or a bezel solitaire. Claws suit stones you want to catch as much light as possible; a bezel protects the edges and suits active hands. You can see design ideas in our page on alexandrite engagement ring designs and the broader guide to engagement rings with coloured gemstones.

A smaller stone also means a slimmer ring, which some people find catches less. If you plan to wear it every day, a comfort-fit band with a rounded inside can make a difference to how it feels over a long day.

How much does alexandrite cost in Australia?

Alexandrite has one of the widest price spreads of any gem. A small natural stone with a weak change can cost less than a good amethyst ring, while a clean, strong-change stone over 1 carat is priced in the same territory as fine ruby. Price per carat climbs steeply with size, because strong-change rough large enough for a 1 carat stone is scarce.

These are broad ranges only; prices vary with the market, the exact change and the report. The biggest price drivers, in order, are strength of change, purity of the two colours, size and clarity. Cat's-eye alexandrite with a sharp eye and good change sits at the upper end for its size. Like any gem, alexandrite is best chosen because you enjoy wearing it, not as a financial asset. For a finished ring, the setting, metal and side stones add to the stone price; our guide to alexandrite rings and their costs covers the whole piece.

How do you clean and care for alexandrite jewellery?

Alexandrite's colour is stable. Its change comes from chromium built into the crystal, not from a treatment or a fragile colour centre, so it does not fade in sunlight the way amethyst can fade after long sun exposure. Most alexandrite is also untreated, though some stones have surface-reaching fractures filled with oil or resin, which should be disclosed.

• Everyday cleaning: warm water, a drop of mild dishwashing liquid and a soft toothbrush, then rinse and dry with a lint-free cloth.
• Ultrasonic and steam: usually safe for untreated, unfractured stones. Avoid both if the stone has fractures or fillings, and ask your jeweller if you are unsure.
• Storage: keep it apart from diamonds and sapphires, which can scratch it, and from softer gems such as pearl and opal, which it can scratch.
• Knocks: alexandrite is tough, but a hard blow on a thin edge can still chip it. Take rings off for gym work, gardening and heavy lifting.
• Check-ups: have the claws or bezel checked about once a year, especially on small stones in fine settings.

How do you choose an alexandrite? Step by step

1. Decide natural or lab-created. Both are real chrysoberyl. Natural is rare and priced accordingly; lab-created gives a strong change at a much lower price and must be labelled lab-created or synthetic.
2. See both colours. Ask for the stone, or a video, under daylight or cool light and under warm incandescent or warm-white LED light, with a white card in view.
3. Judge the strength of change. Look for a clear switch with little in between, rather than a gentle shift in tint.
4. Check the colours themselves. Bluish green and purplish red are preferred over yellowish or brownish tones.
5. Read the report. For a natural stone, confirm the lab names it alexandrite, describes the colour change, and notes any fracture filling or origin opinion.
6. Look at clarity and cut. Aim for a stone that is bright and evenly coloured face-up, without obvious inclusions near the edges.
7. Plan the setting. Choose claws, bezel or halo to suit the stone's size and how you wear your jewellery, and keep the metal colour in mind, as yellow gold warms the red side.