Star Spinel | Gemopedia

Star spinel exhibits the optical phenomenon called asterism, a star-like pattern created on the surface of a gemstone when light encounters parallel fibrous, or needle-like, inclusions within its crystal structure. Light that strikes the inclusions within the gem reflects off of the inclusions, creating a narrow band of light. When two or more intersecting bands appear, a star pattern is formed. Depending on the crystal, the star may have four, six, or even twelve rays. When only one band forms, it is classified as a "cat's eye". Star spinel is highly treasured by collectors for its phenomenal quality.
Colors
Red, Violet, Purple

Star Spinel Classification

Common Name

Star Spinel

Species

Spinel

Variety

Star Spinel

Colors

Red, Violet, Purple

Alternate Names

Gemstone Groups

Key Separations

Appearance, RI and SG

Comments

Star Spinel Optical Properties

Transparency

Transparent - Opaque

Refractive Index

1.718
Tolerance:(+0.017/-0.008)

Birefringence

Optic Character

NA

Optic Sign

NA

Polariscope Reaction

Singly Refractive (SR)

Fluorescence

SWUV: Variable
LWUV: Variable

CCF Reaction

Pleochroism

None

Dispersion

Strength: moderate fire Value: 0.02

Comments

Star Spinel Chemistry & Crystallography

Chemical Name

magnesium aluminum oxide

Chemical Formula

MgAl2O4

Synthesis

Crystal System

Cubic

Classification

Oxide

Nature

Natural

Crystallinity

Crystalline to Aggregate

Comments

Star Spinel Characteristic Physical Properties

Hardness

8

Streak

White

Specific Gravity

3.57-3.7 Typical:3.6

Toughness

Varies

Inclusions

Star spinels will contain needles that may show six or four rayed star. In stones cut as spheres it might be possible to see both four and six rayed stars. Stones might contain natural mineral inclusions like spinel or apatite, negative crystals in the shape of an octahedral and fingerprints.

Luster

SubAdamantine

Stability

Very Good

Fracture

Conchoidal

Cleavage

None

Comments

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