Star Sapphire | Gemopedia

Star sapphire 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 typically has six rays, but on occasion, twelve rays.
Colors
Blue, Black, Brown, Green, Purple, Plum, Rarely Orange, Yellow; If Any Other Color Than Blue Designate By Color

Star Sapphire Classification

Common Name

Star Sapphire

Species

Corundum

Variety

Star Sapphire

Colors

Blue, Black, Brown, Green, Purple, Plum, Rarely Orange, Yellow; If Any Other Color Than Blue Designate By Color

Alternate Names

Gemstone Groups

Key Separations

appearance, RI and magnification

Comments

Stone usually cut as cabochons. Black star sapphires cover black dark brown, green and dark blue.

Star Sapphire Optical Properties

Transparency

Semitransparent - Opaque

Refractive Index

1.762-1.77
Tolerance:(+0.009/-0.005)

Birefringence

0.008-0.01

Optic Character

Uniaxial

Optic Sign

Negative

Polariscope Reaction

Doubly Refractive (DR)

Fluorescence

SWUV: Inert to strong red
LWUV: Inert to strong red

CCF Reaction

Pleochroism

Unobservable

Dispersion

Strength: weak fire Value: 0.018

Comments

Star Sapphire Chemistry & Crystallography

Chemical Name

aluminum oxide

Chemical Formula

Al2O3

Synthesis

Crystal System

Hexagonal

Classification

Oxide

Nature

Natural

Crystallinity

Crystalline

Comments

Star Sapphire Characteristic Physical Properties

Hardness

9

Streak

White

Specific Gravity

4.1-3.95 Typical:4

Toughness

Excellent

Inclusions

Star sapphires will have silk or sets of parallel rutile needles that produce a 6-ray star, hexagonal growth lines, color zoning and mineral inclusions. Weak and less well formed stars along with weaker body color is typical of natural stones. In natural stones at least one ray of the start will be perpendicular to the hexagonal growth zones. Stone with 12-rayed stars are rare but typically occur in dark blue or black stones.

Luster

SubAdamantine

Stability

Very Good

Fracture

Conchoidal

Cleavage

None

Comments

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