Chrysoberyl is a naturally occurring compound highly valued in mineralogy and gemology. It is classified as a mineral species: a naturally occurring, inorganic solid with a defined chemical composition and an ordered atomic structure. Its name, derived from the Greek words for “golden” and “beryl,” hints at its typical coloration, though it is distinct from beryl. Chrysoberyl is prized for its remarkable hardness and unique optical properties, making it a sought-after material for jewelry and collectors for centuries.
Classification and Chemical Composition
Chrysoberyl is formally classified as an oxide mineral, a compound where oxygen is bonded to other elements. The chemical formula is BeAl₂O₄. The presence of beryllium, a relatively rare element in the Earth’s crust, makes chrysoberyl geologically uncommon. This atomic arrangement places the mineral within the broader group of aluminates.
The internal structure is organized into the orthorhombic crystal system, defined by three mutually perpendicular axes of different lengths. The mineral often forms tabular or short prismatic crystals in nature. Trace elements, such as iron, can substitute for aluminum atoms within this lattice, which is responsible for the mineral’s typical coloration.
Defining Physical and Optical Properties
The durability of chrysoberyl is one of its most notable characteristics, ranking at a high 8.5 on the Mohs scale of mineral hardness. This positions it as the third-hardest natural gemstone, behind only diamond and corundum, making it highly resistant to scratching and suitable for daily wear jewelry. Its density is relatively high, with a specific gravity ranging between 3.5 and 3.84.
The mineral typically exhibits a vitreous, or glass-like, luster when polished, which contributes to its brilliance. Common specimens display colors ranging from pale yellow to greenish-yellow and brownish-yellow, created by iron impurities. When crystals break, they display distinct cleavage in one direction and an uneven to conchoidal fracture in others.
Distinct Gemstone Varieties
Alexandrite
The color-changing variety is known as Alexandrite, which exhibits a remarkable shift from a green or bluish-green hue in daylight to a red or purplish-red color under incandescent light. This dramatic transformation, often described as “emerald by day, ruby by night,” is caused by trace amounts of chromium ions substituting for aluminum in the crystal structure. The chromium ions selectively absorb light wavelengths in the yellow part of the spectrum.
When exposed to daylight, which is rich in blue and green light, the stone appears green. Under incandescent light, which contains more red wavelengths, the stone reflects the red light that is not absorbed, causing the color to shift.
Cymophane (Cat’s Eye)
The second major variety is Cymophane, commonly known as Cat’s Eye chrysoberyl, distinguished by a phenomenon called chatoyancy. This effect manifests as a sharp, silky band of light that seems to glide across the surface of the stone when it is moved. Chatoyancy is caused by numerous fine, needle-like inclusions, often rutile, that are aligned perfectly parallel to one another within the crystal structure.
For the cat’s eye effect to be properly displayed, the rough mineral must be precisely cut into a smooth, dome-shaped form called a cabochon. The parallel inclusions must be oriented perpendicularly to the dome’s base. Cat’s Eye chrysoberyl is so well-known for this effect that it is the only gemstone that can be referred to simply as “Cat’s Eye” without any qualifying mineral name.
Geological Formation and Occurrence
Chrysoberyl forms in a high-temperature environment, typically associated with the late-stage crystallization of silica-rich magmas. Its formation commonly occurs within granite pegmatites, which are coarse-grained igneous rocks. The presence of the rare element beryllium is a prerequisite for chrysoberyl to form, as it concentrates in these fluid-rich magmatic pockets.
The mineral can also be found in certain metamorphic rocks, such as mica schists and dolomitic marbles. This occurs when beryllium and aluminum-rich fluids from neighboring pegmatites react with the surrounding rock. Because of its resistance to chemical weathering, chrysoberyl is often found in alluvial deposits, which are river sands and gravels. Major global sources include Brazil, Sri Lanka, Russia, and India, with many gem-quality specimens recovered from these secondary deposits.