Chrysoberyl: Exotic and Expensive Gemstone

Chrysoberyl is a rare and precious gemstone, primarily composed of beryllium and aluminum the formula BeAl₂O₄. Chrysoberyl is found in a variety of colors, including yellow, green, and brown. It is also found in a chatoyant variety called cat's eye chrysoberyl, which has a silky, chatoyant appearance.

Chrysoberyl is a hard gemstone, with a Mohs hardness of 8.5. This makes it durable and resistant to scratches. Chrysoberyl is also a relatively dense gemstone, with a specific gravity of 3.72.

Despite the similarity of their names, chrysoberyl and beryl are two completely different gemstones, although they both contain beryllium. The name "chrysoberyl" is derived from the Greek words χρυσός (chrysos), meaning "gold," and βήρυλλος (beryllos), meaning "spar." This is due to the fact that the mineral was originally mistaken for beryl, another type of gemstone.

Chrysoberyl Formation

Chrysoberyl forms in a variety of geological environments, but it is most commonly found in pegmatites, which are coarse-grained igneous rocks that form from the late-stage crystallization of magma.

Pegmatites are typically found in areas where there has been a lot of tectonic activity, such as mountain ranges and continental margins. The high temperatures and pressures associated with tectonic activity can cause rocks to melt, and the resulting magma can rise towards the surface of the Earth. As the magma cools and crystallizes, it can form a variety of different minerals, including chrysoberyl.

Chrysoberyl also forms in metamorphic rocks, which are rocks that have been changed by heat, pressure, or chemical fluids. Metamorphic rocks can be formed when rocks are buried deep underground, where they are subjected to high temperatures and pressures. These conditions can cause the rocks to recrystallize, and chrysoberyl can form as a result of this process.

It is mined in a number of countries around the world, including Brazil, Sri Lanka, and Russia.

Properties of Chrysoberyl

Chrysoberyl belongs to the orthorhombic crystal system, forming with tabular crystals in slender prisms. Crystal twins and triplets are quite common. Common chrysoberyl is colored by iron, while color change varieties obtain their color through chromium.

• Hardness: 8.5 on the Mohs scale (second only to diamond)
• Specific gravity: 3.7
• Luster: vitreous (glassy)
• Color: Chrysoberyl can be found in a variety of colors, including yellow, green, red, orange, and colorless. Alexandrite, a rare variety of chrysoberyl, exhibits a color change from green in daylight to red or purplish red under incandescent light.
• Transparency: Transparent to translucent
• Cleavage: Poor cleavage
• Crystal structure: Orthorhombic

Appearance and Colors Of Chrysoberyl

Common chrysoberyl occurs in a variety of light colors, including green to yellow and golden-yellow to yellowish-green, along with various shades of brown and red. Chrysoberyl is colored by iron, while color change chrysoberyl (alexandrite) is colored by chromium. Some 'common' chrysoberyl may also contain chromium, but unless they exhibit color change, they are not traded as alexandrite.

An interesting feature of chrysoberyl 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. 

Chrysoberyl crystals From Sri Lanka. Photo: City Of Gem

Unique properties of chrysoberyl

• High hardness: Chrysoberyl is one of the hardest gemstones, with a Mohs hardness of 8.5 to 9. This makes it a durable gemstone that is ideal for jewelry.
• Chatoyancy: Some varieties of chrysoberyl, such as cymophane, exhibit chatoyancy. This is a shimmering effect that occurs when light reflects off of microscopic fibers in the mineral.
• Dichroism: Alexandrite is a dichroic gemstone, which means that it appears to change color in different lighting conditions. In daylight, alexandrite appears green, while in incandescent light, it appears red.

Chrysoberyl Varieties

Chrysoberyl varieties include two of the world's most exotic and expensive gems, alexandrite and cat's-eye chrysoberyl.

Alexandrite effect

Alexandrite

Alexandrite, the most renowned variety of chrysoberyl, is renowned for its mesmerizing color-changing ability. Alexandrite will exhibit emerald green, red and orange-yellow colors and tend to change color in artificial light compared to daylight. The color change from red to green is due to strong absorption of light in the yellow and blue portions of the spectrum. Typically, alexandrite has an emerald green color in daylight but exhibit a raspberry red color in incandescent light. 

The color-changing ability of alexandrite is not just a visual spectacle; it is a testament to the gemstone's intricate atomic arrangement. As light interacts with the chromium ions, it is absorbed and re-emitted at a different wavelength, causing the stone to appear green under daylight and red under incandescent light. This phenomenon, known as chromaticism, is a fascinating example of how the composition and structure of a material can influence its optical properties.

Chrysoberyl specimen with " star " form. Collection D.Preite photo M.Chinellato CC

Alexandrite, named after Tsar Alexander II of Russia, is a highly sought-after gemstone, captivating collectors and enthusiasts worldwide. Its rarity, captivating color change, and exceptional hardness make it a coveted treasure among gemstone enthusiasts.

Cymophane or Cat's-eye Chrysoberyl

Cymophane is popularly known as cat´s eye. This variety exhibits pleasing chatoyancy or opalescence that reminds one of an 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.

This optical phenomenon is caused by parallel fibers of rutile inclusions within the crystal. As light interacts with these fibers, it is scattered and reflected, creating a single, shimmering band of light that moves across the stone as it is rotated in different directions.

Chrysoberyl Uses

Chrysoberyl is a rare and precious mineral with a variety of uses. It is most commonly used as a gemstone, but it also has some industrial applications.

Chrysoberyl sixling twin from Okkampitiya, Sri Lanka Photo: Thomas Spann

Gemstone Uses

Chrysoberyl is a popular gemstone due to its hardness, durability, and wide range of colors. The most common varieties of chrysoberyl used in jewelry are alexandrite, cat's eye, and ordinary yellow-to-green chrysoberyl.

Industrial Uses

Chrysoberyl has some industrial applications, but these are less common than its gemstone uses. Some industrial applications of chrysoberyl include:

• Cutting tools
• Bearings

Overall, chrysoberyl is a versatile mineral with a variety of uses. It is most commonly used as a gemstone, but it also has some industrial applications.