Chrysoberyl

The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula BeAl2O4. The name chrysoberyl is derived from the Greek words χρυσός chrysos and βήρυλλος beryllos, meaning "a gold-white spar". Despite the similarity of their names, chrysoberyl and beryl are two completely different gemstones, although they both contain beryllium. Chrysoberyl is the third-hardest frequently encountered natural gemstone and lies at 8.5 on the Mohs scale of mineral hardness, between corundum (9) and topaz (8).

An interesting feature of its crystals are the cyclic twins called trillings. These twinned crystals have a hexagonal appearance, but are the result of a triplet of twins with each "twin" oriented at 120° to its neighbors and taking up 120° of the cyclic trilling. If only two of the three possible twin orientations are present, a V-shaped twin results.

Ordinary chrysoberyl is yellowish-green and transparent to translucent. When the mineral exhibits good pale green to yellow color and is transparent, then it is used as a gemstone. The three main varieties of chrysoberyl are: ordinary yellow-to-green chrysoberyl, cat's eye or cymophane, and alexandrite. Yellow-green chrysoberyl was referred to as "chrysolite" during the Victorian and Edwardian eras, which caused confusion since that name has also been used for the mineral olivine ("peridot" as a gemstone); that name is no longer used in the gemological nomenclature.

Alexandrite, a strongly pleochroic (trichroic) gem, will exhibit emerald green, red and orange-yellow colors depending on viewing direction in partially polarised light. However, its most distinctive property is that it also changes color in artificial (tungsten/halogen) light compared to daylight. The color change from red to green is due to strong absorption of light in a narrow yellow portion of the spectrum, while allowing large bands of more blue-green and red wavelengths to be transmitted. Which of these prevails to give the perceived hue depends on the spectral balance of the illumination. Fine-quality alexandrite has a green to bluish-green color in daylight (relatively blue illumination of high color temperature), changing to a red to purplish-red color in incandescent light (relatively yellow illumination). However, fine-color material is extremely rare. Less-desirable stones may have daylight colors of yellowish-green and incandescent colors of brownish red.

Cymophane is popularly known as "cat's eye". This variety exhibits pleasing chatoyancy or opalescence that reminds one of the eye of a cat. When cut to produce a cabochon, the mineral forms a light-green specimen with a silky band of light extending across the surface of the stone.

Chrysoberyl

General

Category Oxide minerals

Formula BeAl2O4

IMA symbol Cbrl

Strunz classification 4.BA.05

Crystal system Orthorhombic

Crystal class Dipyramidal (mmm)

H-M symbol: (2/m 2/m 2/m)

Space group Pbnm

Unit cell a = 5.481 Å,

b = 9.415 Å,

c = 4.428 Å; Z = 4

Identification

Color Various shades of green, emerald-green yellow, blue, brownish to greenish black, may be raspberry-red under incandescent light when chromian; colorless, pale shades of yellow, green, or red in transmitted light

Crystal habit Crystals tabular or short prismatic, prominently striated

Twinning Contact and penetration twins common, often repeated forming rosette structures

Cleavage Distinct on {110}, imperfect on {010}, poor on {001}

Fracture Conchoidal to uneven

Tenacity Brittle

Mohs scale hardness 8.5

Luster Vitreous

Streak White

Specific gravity 3.5–3.84

Optical properties Biaxial (+)

Refractive index nα=1.745 nβ=1.748 nγ=1.754

Pleochroism X = red; Y = yellow-orange; Z = emerald-green

2V angle Measured: 70°

References

Major varieties

Alexandrite Color change; green to red

Cymophane Chatoyant

Etymology and history

The name chrysoberyl comes from the Greek χρυσοβήρυλλος (chrysobḗryllos), which is composed of the words χρυσός (chrysós) for “gold” and βήρυλλος (bḗryllos) for “ beryl ”.

Chrysoberyl is one of the approximately 20 gemstones described by the Roman writer Pliny the Elder (c. 23–79 AD) in his Naturalis Historia. Pliny mistakenly considered chrysoberyl to be a subspecies of beryl, a golden-colored brother of aquamarine (blue) and emerald (green), to which it does not belong—although it also contains beryllium cations—and differs from them in chemical composition, structure, and hardness. Nevertheless, chrysoberyl was not listed as an independent mineral (crisoberil) until 1789 in Abraham Gottlob Werner 's mineral classification. 

Classification

In the outdated 8th edition of the mineral classification according to Strunz, chrysoberyl belonged to the mineral class of “oxides and hydroxides” and there to the section “compounds with M 3 O 4 and related compounds”, where it is listed together with taaffeite group and swedenborgite in the “chrysoberyl-swedenborgite group” with the system number IV/B.04.

In the Lapis classification system by Stefan Weiß, last revised in 2018 and formally based on the 8th edition of Karl Hugo Strunz 's old classification system, the mineral was given the system and mineral number IV/B.07-010. This corresponds to the class of "Oxides and Hydroxides" and the section "Oxides with a metal:oxygen ratio of 3:4 (spinel type M3O4 and related compounds)", where chrysoberyl forms an unnamed group with the system number IV/B.07 together with ferroaffeite, magnesiotaaffeite, mariinskite, and swedenborgite. 

The 9th edition of Strunz's mineral classification, last updated in 2009 by the International Mineralogical Association (IMA), classifies chrysoberyl in the class of "Oxides (Hydroxides, V -Vanadates, Arsenites, Antimonites, Bismuthites, Sulfites, Selenites, Tellurites, Iodates)" and there in the division "Metal: Oxygen = 3: 4 and comparable". Here, the mineral can be found in the subdivision "With small and medium-sized cations", where it is the only member of an unnamed group with the system number 4.BA.05.

In the Dana classification of minerals, which is primarily used in English-speaking countries, chrysoberyl has the system and mineral number 07.02.09.01. This corresponds to the class "Oxides and Hydroxides" and the division "Multiple Oxides." Here, it is found within the subdivision "Multiple Oxides (A + B 2+) 2 X 4, Spinel Group" as the sole member of an unnamed group with the system number 07.02.09.

Crystal structure

Chrysoberyl crystallizes orthorhombic in the space group Pbnm (space group no. 62, position 3) with the lattice parameters a = 4.43 Å; b = 9.40 Å and c = 5.47 Å and 4 formula units per unit cell. 

The crystal structure is similar to that of olivine, but unlike olivine, it consists of tetrahedra, whose vertices are linked by octahedrally coordinated Al 3+ atoms. The crystal structure can therefore be described analogously to olivine as Al 2. 

Characteristics

Chrysoberyl is very sensitive to various alkalis and potassium hydrogen sulfate (potassium bisulfate) and is decomposed by them. However, it is not affected by blowpipes or acids. 

Varieties and modifications

Alexandrite, a very rare and valuable variety (chrysoberyl- mariinskite mixed series), shimmers green to bluish green in daylight and red to violet under artificial light. This color change, also called iridescence or alexandrite effect, is caused by its chromium content. The reason is two spectral regions with low absorption (high light transmission) and a region in between with strong absorption together with the different spectral brightness maximum of daylight and artificial light. In daylight, which contains a larger proportion of green light, it therefore appears green. In incandescent lamp or candlelight, whose red component is much stronger than the green, it appears strong red. In addition, alexandrite shows direction-dependent color change - pleochroism.

The name alexandrite goes back to the future Russian Tsar Alexander II (reigned 1855–1881), on whose occasion of his adulthood, Lev Alexeyevich Perovsky named the stone in his honor. It had previously been studied by Nils Gustaf Nordenskiöld. The main colors of the Russian army at that time were green and red.

Another variety is chrysoberyl cat's eye, or simply cat's eye (obsolete and no longer used synonyms are cymophane or kymophane), which exhibits the coveted cat's eye effect. Only this variety may bear the exclusive designation "cat's eye." All other minerals exhibiting the cat's eye effect must be identified by the corresponding mineral name. The undulating, silvery-white light streak is created by the refraction of light in the fine, parallel hollow channels.

Alexandrite

The alexandrite variety displays a color change dependent upon the nature of ambient lighting. Alexandrite results from small scale replacement of aluminium by chromium ions in the crystal structure, which causes intense absorption of light over a narrow range of wavelengths in the yellow region (520–620 nm) of the visible light spectrum. Because human vision is most sensitive to green light and least sensitive to red light, alexandrite appears greenish in daylight where the full spectrum of visible light is present, and reddish in incandescent light which emits less green and blue light. This color change is independent of any change of hue with viewing direction through the crystal that would arise from pleochroism.

Alexandrite from the Ural Mountains in Russia can be green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light.

Stones that show a dramatic color change and strong colors (e.g., red-to-green) are rare and sought-after, but stones that show less distinct colors (e.g. yellowish green changing to brownish yellow) may also be considered "alexandrite" by gem labs such as the Gemological Institute of America.

According to a popular but controversial story, alexandrite was discovered by the Finnish mineralogist Nils Gustaf Nordenskiöld (1792–1866), and named alexandrite in honor of the future Emperor of All Russia Alexander II Romanov. Nordenskiöld's initial discovery occurred as a result of an examination of a newly found mineral sample he had received from Perovskii, which he identified as emerald at first. The first emerald mine had been opened in 1831. However, recent research suggests that the stone was discovered by Yakov Kokovin.

Alexandrite 5 carats (1,000 mg) and larger were traditionally thought to be found only in the Ural Mountains, but have since been found in larger sizes in Brazil. Other deposits are located in India (Andhra Pradesh), Madagascar, Tanzania and Sri Lanka. Alexandrite in sizes over three carats are very rare. Today, several labs can produce synthetic lab-grown stones with the same chemical and physical properties as natural alexandrite. Several methods can produce flux-grown alexandrite, Czochralski (or pulled) alexandrite, and hydrothermally-produced alexandrite. Flux-grown gems are fairly difficult to distinguish from natural alexandrite as they contain inclusions that seem natural. Czochralski or pulled alexandrite is easier to identify because it is very clean and contains curved striations visible under magnification. Although the color change in pulled stones can be from blue to red, the color change does not truly resemble that of natural alexandrite from any deposit. Hydrothermal lab-grown alexandrite has identical physical and chemical properties to real alexandrite.

Some gemstones falsely described as lab-grown synthetic alexandrite are actually corundum laced with trace elements (e.g., vanadium) or color-change spinel and are not actually chrysoberyl. As a result, they would be more accurately described as simulated alexandrite rather than "synthetic". This alexandrite-like sapphire material has been around for almost 100 years and shows a characteristic purple-mauve colour change, which does not really look like alexandrite because there is never any green.

Alexandrite is also known as one of three official birthstones for the month of June.

Cymophane

Translucent yellowish chatoyant chrysoberyl is called cymophane or cat's eye. Cymophane has its derivation also from the Greek words meaning 'wave' and 'appearance', in reference to the haziness that visually distorts what would normally be viewed as a well defined surface of a cabochon. This effect may be combined with a cat eye effect. In this variety, microscopic tubelike cavities or needle-like inclusions of rutile occur in an orientation parallel to the c-axis, producing a chatoyant effect visible as a single ray of light passing across the crystal. This effect is best seen in gemstones cut in cabochon forms perpendicular to the c-axis. The color in yellow chrysoberyl is due to Fe3+ impurities.

Although other minerals such as tourmaline, scapolite, corundum, spinel and quartz can form "cat's eye" stones similar in appearance to cymophane, the jewelry industry designates these stones as "quartz cat's eyes", or "ruby cat's eyes" and only chrysoberyl can be referred to as "cat's eye" with no other designation.

Gems lacking the silky inclusions required to produce the cat's eye effect are usually faceted. An alexandrite cat's eye is a chrysoberyl cat's eye that changes color. "Milk and honey" is a term commonly used to describe the color of the best cat's eyes. The effect refers to the sharp milky ray of white light normally crossing the cabochon as a center line along its length and overlying the honey-colored background. The honey color is considered to be top-grade by many gemologists but the lemon yellow colors are also popular and attractive. Cat's eye material is found as a small percentage of the overall chrysoberyl production wherever chrysoberyl is found.

Cat's eye became significantly more popular by the end of the 19th century when the Duke of Connaught gave a ring with a cat's eye as an engagement token; this was sufficient to make the stone more popular and increase its value greatly. Until that time, cat's eye had predominantly been present in gem and mineral collections. The increased demand in turn created an intensified search for it in Sri Lanka.

Occurrence

Chrysoberyl forms as a result of pegmatitic processes. Melting in the Earth's crust produces relatively low-density molten magma which can rise upwards towards the surface. As the main magma body cools, water originally present in low concentrations became more concentrated in the molten rock because it could not be incorporated into the crystallization of solid minerals. The remnant magma thus becomes richer in water, and also in rare elements that similarly do not fit in the crystal structures of major rock-forming minerals. The water extends the temperature range downwards before the magma becomes completely solid, allowing concentration of rare elements to proceed so far that they produce their own distinctive minerals. The resulting rock is igneous in appearance but formed at a low temperature from a water-rich melt, with large crystals of the common minerals such as quartz and feldspar, but also with elevated concentrations of rare elements such as beryllium, lithium, or niobium, often forming their own minerals; this is called a pegmatite. The high water content of the magma made it possible for the crystals to grow quickly, so pegmatite crystals are often quite large, which increases the likelihood of gem specimens forming.

Chrysoberyl can also grow in the country rocks near to pegmatites, when Be- and Al-rich fluids from the pegmatite react with surrounding minerals. Hence, it can be found in mica schists and in contact with metamorphic deposits of dolomitic marble. Because it is a hard, dense mineral that is resistant to chemical alteration, it can be weathered out of rocks and deposited in river sands and gravels in alluvial deposits with other gem minerals such as diamond, corundum, topaz, spinel, garnet, and tourmaline. When found in such placers, it will have rounded edges instead of sharp, wedge-shape forms. Much of the chrysoberyl mined in Brazil and Sri Lanka is recovered from placers, as the host rocks have been intensely weathered and eroded.

If the pegmatite fluid is rich in beryllium, crystals of beryl or chrysoberyl could form. Beryl has a high ratio of beryllium to aluminium, while the opposite is true for chrysoberyl. Both are stable with the common mineral quartz. For alexandrite to form, some chromium would also have had to be present. However, beryllium and chromium do not tend to occur in the same types of rock. Chromium is most common in mafic and ultramafic rocks in which beryllium is extremely rare. Beryllium becomes concentrated in felsic pegmatites in which chromium is almost absent. Therefore, the only situation where an alexandrite can grow is when Be-rich pegmatitic fluids react with Cr-rich country rock. This unusual requirement explains the rarity of this chrysoberyl variety.

Use as a gemstone

Chrysoberyl and its varieties are primarily used as gemstones, but only a small portion of chrysoberyl crystals are clear and transparent, as required for jewelry making. Usually, only relatively small pieces of the crystal can be cut and polished into clear, lustrous, and warmly luminous "gemstones," using various facet cuts. Cat's eye, on the other hand, receives the cabochon cut necessary to optimally emphasize the chatoyancy.

Due to similarities in color and shape, chrysoberyl can be confused with various other minerals, some of which are also used in jewelry, such as andalusite, brazilianite, golden beryl, hiddenite, peridot, sapphire, sinhalite, scapolite, spinel, topaz, tourmaline, and zircon. 

Famous chrysoberyls

The largest chrysoberyl found to date was found in Rio de Janeiro, Brazil, and weighed 16 pounds. Another very large stone, weighing 1876 ct (≙ 375.2 g), was found in Sri Lanka. 

The largest cut alexandrite known to date weighs 66 ct and is housed in the Smithsonian Institution in Washington (USA). Also famous is the "Hope Chrysoberyl," a light green, faceted stone weighing 45 ct, housed in London. 

Manipulations and imitations

Since chrysoberyl and especially the extremely rare and expensive alexandrite are rare and correspondingly expensive gemstones, they are often imitated using various methods:

Alexandrite has also been produced synthetically since 1888. These synthetic crystals can only be clearly distinguished from natural stones through gemmological examination. Inclusions play an important role in this process.

Similar, cheaper minerals such as cat's-eye quartz are often used to imitate chrysoberyl. Other imitations are created using glass, synthetic corundum, or spinel. Synthetic corundum, preferably sapphire, is also used to imitate alexandrite because it exhibits a similar color change, albeit more from red to violet. The trade names Blue Alexandrite and Sri Lankan Alexandrite are therefore actually sapphires.

Very successful imitations of chrysoberyl are achieved by creating doublets (composite gemstones) with garnet or glass as the base.

In order to enhance the natural chrysoberyls with less valuable color characteristics by changing or intensifying the color, they have been subjected to radioactive irradiation since 1997. However, since strong residual radiation is produced, especially when irradiated with elementary particles, the stones treated in this way sometimes have to be quarantined for several years.

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