Tourmaline (Cat's-Eye) | Gemopedia

The term cat's eye, or chatoyancy, is used to describe a phenomenal optical property in gemstones, in this case tourmaline. The effect, when present, appears as a bright, narrow slit similar to the pupils in the eyes of your favorite feline. This phenomenon is caused by parallel fibrous or needle-like inclusions that interfere with the passage of light throughout the crystal, scattering and reflecting light back to the viewer as a thin line. Cat's eye tourmaline can occur in many colors but particularly in green and pink varieties.
Colors
Most Often Dark Bluish Green To Dark Greenish Blue But Can Be Any Color

Tourmaline (Cat's-Eye) Classification

Common Name

Tourmaline (Cat's-Eye)

Species

Elbaite

Variety

Cat's-Eye Tourmaline

Colors

Most Often Dark Bluish Green To Dark Greenish Blue But Can Be Any Color

Alternate Names

Gemstone Groups

Tourmaline

Key Separations

appearance, refractive index and high birefringence; then optic figure, pleochroism and specific gravity

Comments

Not all cat's-eye tourmaline is elbaite Na(Li1.5Al1.5)Al6(Si6O18)(BO3)3(OH)3(OH). Dark material might have birefringence as high as 0.040. Pyroelectric: development of opposite charges at the ends of a tourmaline crystal due to change in temperature. Piezoelectric: development of opposite charges at the ends of a tourmaline crystal when placed under stress.

Tourmaline (Cat's-Eye) Optical Properties

Transparency

Transparent - Translucent

Refractive Index

1.624-1.644
Tolerance:-1.22222222222222

Birefringence

0.018-0.02

Optic Character

Uniaxial

Optic Sign

Negative

Polariscope Reaction

Doubly Refractive (DR)

Fluorescence

SWUV: Variable Pink: inert to very weak red to violet
LWUV: Variable Pink: inert to very weak red to violet

CCF Reaction

Pleochroism

Dichroic, medium to strong variations of bodycolor

Dispersion

Strength: moderate fire Value: 0.017

Comments

Tourmaline (Cat's-Eye) Chemistry & Crystallography

Chemical Name

complex boro-silicate of Al, Mg and Fe

Chemical Formula

Na(Li1.5Al1.5)Al6(Si6O18)(BO3)3(OH)3(OH)

Synthesis

Crystal System

Trigonal

Classification

Silicate

Nature

Natural

Crystallinity

Crystalline

Comments

Tourmaline (Cat's-Eye) Characteristic Physical Properties

Hardness

7-7.5

Streak

White

Specific Gravity

3-3.26 Typical:3.06

Toughness

Good

Inclusions

Cat's-eye tourmaline has Type III clarity. Stones has numerous parallel hollow tubes. The eye is less focused and harder to see than other cat's-eye stones. Stones might have liquid and gas inclusions that are long and thin, reflective gas-filled fractures and color zoning.

Luster

Vitreous

Stability

Very Good

Fracture

Conchoidal, Subconchoidal, Irregular, Uneven

Cleavage

Poor, in two directions

Comments

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