Alexandrite

gemology

Alexandrite

Alexandrite

Name Alexandrite
Is a Variety of Chrysoberyl
Crystallography Orthorhombic
Refractive Index 1.745–1.759
Colors Varies in color with incident light: green, blue-green, or pale green in daylight; mauve, violet to red, purplish in incandescent light.
Luster Vitreous.
Hardness 8.5
Wearability Excellent
Fracture Weak conchoidal to uneven.
Specific Gravity 3.68–3.80
Birefringence 0.009–0.010
Cleavage Distinct to poor, 1 direction.
Dispersion 0.015
Heat Sensitivity Yes.
Luminescence Weak red in SW and LW.
Luminescence Present Yes
Luminescence Type Fluorescent, UV-Long, UV-Short
Special Care Instructions None
Transparency Opaque to transparent.
Absorption Spectrum Narrow doublet at 6805/6875, with weak, narrow lines at 6650, 6550, and 6490 and broad band at 6400-5550. Total absorption below 4700.
Phenomena Color change, chatoyancy (very rare).
Birthstone June
Formula BeAl2O4 + Cr
Pleochroism Deep red/orange-yellow/green. (Myanmar gems anomalous: purple/grass-green/blue-green).
Optics See “Identifying Characteristics” below. Biaxial (+), may also be (–), 2V = 70°.
Optic Sign Biaxial +, Biaxial –
Etymology Named after Czar Alexander II of Russia.
Occurrence Occurs in pegmatites, gneiss, mica schist, dolomitic marbles; also found as stream pebbles and detrital grains.

“Emerald by day, ruby by night,” alexandrite is well known for displaying one of the most remarkable color changes in the gem world.

green in sunlight and red in incandescent light.

However, the modern June birthstone is so rare and expensive few people have seen a natural alexandrite.

This variety of gem-quality chrysoberyl makes an excellent jewelry stone.

Alexandrites have two primary value drivers.

First, the closer the colors to pure green and red, the higher the value.

Second, the more distinct the color change, the higher the value.

Alexandrites can exhibit everything from 100% to just 5% color change.

Thus, the most valuable gems would have a 100% color shift from pure green to pure red.

Blue-greens and purplish or brownish reds hold less value.

Clarity also plays a significant role in grading.

As is the case with a majority of gems, most naturally occurring alexandrite isn’t clean, facetable material.

Most is best suited for cabbing.

However, an alexandrite’s color change has more effect on its value than its clarity.

For example, take two alexandrites of equal size.

One gem is eye clean, with a 50% greenish blue to brownish-red color change.

The other is an opaque cabochon with a 100% green to red color change.

The opaque cab would be considered more valuable.

Size always affects alexandrite value.

You can see this reflected in our Price Guide below.

In sizes up to one carat, top-quality natural gems can sell for up to $15,000 per carat.

Over one carat, the prices range from $50,000 to $70,000 per carat!

Alexandrite was discovered in the Ural Mountains of Russia in the 1830s.

Noted mineralogist Nils Gustaf Nordenskiöld was the first to realize this unusual green, the color-changing gemstone was something new.

In 1834, Count Lev Alekseevich Perovskii named the stone in honor of the then-future Czar of Russia, Alexander II.

This connection to the Czars likely helped the gem gain prestige by association.

In the 19th and early 20thcenturies, as historian David Cannadine notes, the Czars were widely considered the standard for royal pomp. (More recently, the British Royal Family has enjoyed this position).

A combination of beauty, celebrity, and rarity helped create a mystique around this gem in the public imagination.

By the 1950s, alexandrite joined the list of birthstones as the modern alternative to June’s traditional pearl.

How Rare is Alexandrite?

If not for alexandrite’s popular associations, the circumstances necessary for its formation, combined with its mining history, might have ensured the gem would be little known as well as extremely rare.

To form, alexandrite requires both beryllium (Be), one of the rarest elements on Earth, and chromium (Cr). (Emerald also requires these two elements).

However, Be and Cr rarely occurs in the same rocks or in geological conditions where they interact.

Furthermore, the original source of alexandrites was almost exhausted after only a few decades of mining.

Since the 1980s, more sources have emerged. Nevertheless, alexandrite remains one of the rarest gemstones.

Cat’s Eye Alexandrites

Cats Eye AlexandriteAlexandrite is a variety of the gem species chrysoberyl, well-known for its chatoyancy or “cat’s eye” effect when cabbed.

As members of this species, alexandrites can also show a cat’s eye effect.

However, such gems are quite rare.

Identifying Characteristics

Color Change

The color change gemstone phenomenon can occur under a variety of lighting types.

When grading an alexandrite’s color change, gemologists consider the stone’s color in natural sunlight as the baseline.

Thus, the classic alexandrite color change is green in sunlight and red in incandescent light.

However, other types of lights can produce other colors, as shown below.

Alexandrite color changes - J Weyer.

Alexandrite color changes. Photo: J. Weyer.

Regional Variations in Alexandrites

Brazilian alexandrites tend to have pale colors, pale blue-green to pale mauve.

However, finer gems have been found recently in limited quantity.

Gemologists have detected substantial amounts (1,200 ppm) of the element gallium (Ga) replacing aluminum (Al) in some Brazilian materials.

Sri Lankan alexandrite often appears deep olive green in sunlight, whereas Russian stones appear bluish-green in the sunlight.

Zimbabwean gems show a fine, emerald-green color in sunlight but are usually tiny.

If clean, they weigh under 1 carat. The color change in Zimbabwean gems is among the best known, but large, clean stones are virtually unobtainable from the rough from this locality.

Other physical and optical properties of alexandrites vary according to their source.

Synthetics

A considerable market exists for lab-created alexandrite, first synthesized in the 1960s.

Manufacturers can grow alexandrites through melt, hydrothermal, or flux methods.

These synthetic stones have the same chemical and physical properties as natural alexandrites.

They are real alexandrites but not natural.

Although the synthetics cost far less than their natural counterparts, they still rank among the most expensive synthetic gemstones available.

Gemologists can sometimes identify synthetic alexandrites by inclusions caused by various growth procedures.

Melt techniques, like the Czochralski method, can create curved striae.

Hydrothermal growth can create bubbles and liquid inclusions.

Flux methods can leave inclusions of platinum or other seed materials.

A considerable market also exists for lookalikes or simulants.

These can range from synthetic corundum with alexandrite-like color change (produced very inexpensively) to actual, natural color-change chrysoberyl stones.

Although alexandrite is a variety of chrysoberyl, not all color-change chrysoberyls are alexandrites.

These gems also command a high price, but, again, not nearly as high as alexandrites.

Buyer beware. If you find an alexandrite at a relatively bargain price, it’s likely not natural and possibly not an alexandrite.

A professional gemological laboratory can make a determination.

Enhancements

Natural alexandrites usually don’t receive any treatments.

Sources

Mines in the Urals have re-opened but only produce a few carats of gem-quality material each year.

In 1987, alexandrite was discovered in Brazil and later in Madagascar, Myanmar, Sri Lanka, and Zimbabwe.

However, none of these sites produce as rich and vivid a color change as the original Russian source.

The main source of large, natural alexandrite gems today is actually antique jewelry.

Stones Sizes

The largest known faceted alexandrite, a 65.7-ct green/red color change stone from Sri Lanka, resides at the Smithsonian Institution.

The largest Russian gems weigh about 30 carats.

However, the vast majority of alexandrites weigh under one carat.

Stones over five carats are very rare, especially with good color change.

Other alexandrites of notable size included in the following:

  • British Museum of Natural History (London): 43 and 27.5 cts (Sri Lanka).
  • Institute of Mines (St. Petersburg, Russia): a cluster of three crystals, 6 x 3 cm (Urals).
  • Fersman Museum (Moscow, Russia): crystal group, 25 x 15 cm, crystals up to 6 x 3 cm (Urals).
  • Private Collections: stones up to 50cts have been reported.

Care

With a hardness of 8.5, alexandrite makes a very durable stone suitable for any jewelry setting.

Nevertheless, take care when faceting the stone.

Alexandrite is still sensitive to knocks and extreme heat.

These gems have no special care requirements.

You can clean them mechanically, per the instructions of the system used.

Of course, you can also wash them with warm, soapy water and a brush.

gemology

Article Prepared by :

සම්පත් සමරසේකර
Sampath Samarasekara

Chairman: Gemological Institute of Ceylon
Chairman: Youth Gem Professionals Association
Chairman: Sampath Gems
Director: Ceylon Sapphire Gems and Jewels
Director: Ceylon Gem Fair International
Chairman: Nanosoft Web Develop Company

Direct WhatsApp: https://wa.me/message/2A4AUALYVQWRA1

Join with Gemological Institute of Ceylon for Practical Gemology Studies
Facebook Page: https://www.facebook.com/giceylon
Facebook Group: https://www.facebook.com/groups/1990653001208203
Facebook Profile: GI Ceylon

Websites :
www.giceylon.com
www.bluesapphire.lk
www.gemluck.com
www.buyroughgems.com
www.ceylongemfair.com

Join with Gem and Jewellery Business Help and Discussion Group on Telegram :

https://t.me/joinchat/Q5cLnxilNw4xAr8T3TumfA

All images and text content Copyrighted ©️