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Cat's Eye Opal

Cat’s eye opal is a rare variety of opal exhibiting chatoyancy, a distinct optical phenomenon that creates a luminous band of light across its surface, similar to the vertical pupil of a feline eye. This silica-based gemstone forms in metamorphic and sedimentary rocks, containing parallel inclusions of rutile or other minerals that produce the cat’s eye effect when cut en cabochon.

The phenomenon occurs due to the precise alignment of tubular channels within the opal’s microstructure, combined with the play-of-color characteristic of precious opal. These channels, filled with different materials, create a concentrated beam of reflected light perpendicular to their orientation. The most valuable specimens display sharp, well-defined bands against vivid background colors, with specimens from Australia and Brazil commanding premium market prices.

The gemstone measures 5.5-6.5 on the Mohs hardness scale and exhibits a distinctive internal structure that differentiates it from other chatoyant gems like cat’s eye chrysoberyl or quartz. Its unique optical properties make it particularly sought after in both jewelry design and mineralogical collections.

Type of opal (natural vs synthetic)

Cat’s eye opal is typically a natural gemstone, though synthetic versions do exist. The natural versions of this gemstone are more valued and sought after.

Synonyms / common names

Cat’s eye opal is also commonly referred to as cymophane when it exhibits notable chatoyancy.

Hardness

The hardness of cat’s eye opal is typically around 5.5 to 6.5 on the Mohs scale, making it relatively soft and somewhat susceptible to scratches.

Refractive Index (RI)

The Refractive Index of cat’s eye opal ranges from approximately 1.44 to 1.47.

Luster

This type of opal typically has a waxy to resinous luster, which enhances its cat’s eye effect.

Cleavage

Cat’s eye opal has no cleavage, which is a common characteristic of opal in general.

Fracture

The fracture of cat’s eye opal is typically conchoidal, which is a smooth breakage pattern similar to that of glass.

Specific gravity (sg)

The specific gravity of cat’s eye opal typically ranges from about 2.15 to 2.23.

Optical birefringence (double refraction)

Opals, including cat’s eye opal, are isotropic and do not exhibit double refraction.

Dispersion (fire)

Cat’s eye opal has low to moderate dispersion, giving it a subtle play of color or “fire” that is highly prized.

Crystal system

Opal is amorphous, meaning it does not have a crystalline structure.

Color

The body color of cat’s eye opal can vary widely, including white, gray, green, brown, and pink.

Transparency

Cat’s eye opal ranges from opaque to translucent, which affects the visibility of the cat’s eye effect.

Pleochroism

Being isotropic, cat’s eye opal does not exhibit pleochroism.

Fluorescence

Cat’s eye opal may exhibit fluorescence under UV light, typically showing green or white colors.

Toughness

Opal is generally considered to have fair to good toughness, but it can be brittle and sensitive to changes in temperature and humidity.

Tenacity

Cat’s eye opal is brittle, which is typical for opal.

Optic sign

Being isotropic, the optic sign of cat’s eye opal is not applicable.

Absorption spectrum

Cat’s eye opal does not have a characteristic absorption spectrum.

Chemical composition

Opal is a hydrated form of silica (SiO2·nH2O).

Chatoyancy (cat’s eye effect)

The chatoyancy, or cat’s eye effect, in cat’s eye opal is caused by the reflection of light by parallel fibers, needles, or channels within the stone.

Asterism (star effect)

Cat’s eye opal does not typically display asterism.

Iridescence

This type of opal can display iridescence, especially if it has inclusions that diffract light.

Magnetism

Cat’s eye opal is not magnetic.

Electrical conductivity

As a type of opal, cat’s eye opal is an insulator and does not conduct electricity effectively.

Radioactivity

Cat’s eye opal is not radioactive.

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