Sillimanite | Gemopedia

Beautiful and rare, sillimanite is named for the famous American geologist Benjamin Silliman and was relatively unknown until a substantial find was discovered in Orissa, India in the 1990's. Sillimanite is not only scarce, but also difficult for miners to identify and problematic for cutters. These three attributes ensure that sillimanite remains a true exotic gemstone. Sillimanite ranges from colorless to white, brown, yellow, blue and green in color and consists of compact fibrous material that has a silky luster. A polymorph of kyanite and andalusite, sillimanite makes an exciting addition to any gemstone collection.
Colors
Slate-Blue, Blue-Green, Grayish Green, Violet Blue

Sillimanite Classification

Common Name

Sillimanite

Species

Sillimanite

Variety

Colors

Slate-Blue, Blue-Green, Grayish Green, Violet Blue

Alternate Names

Gemstone Groups

Key Separations

RI, SG and fracture, possibly fluorescence and birefringence.

Comments

Sillimanite is one of the most difficult gemstones to fashion.

Sillimanite Optical Properties

Transparency

Transparent - Translucent

Refractive Index

1.659-1.68
Tolerance:(+0.004/-0.006)

Birefringence

0.015-0.021

Optic Character

Biaxial

Optic Sign

Positive

Polariscope Reaction

Doubly Refractive (DR)

Fluorescence

SWUV: inert or weak red
LWUV: inert or weak red

CCF Reaction

Pleochroism

Trichroic, (in blue gems) strong blue, colorless, and light yellow; (in green gems) strong yellowish green, dark green, and blue

Dispersion

Strength: weak fire Value: 0.015

Comments

Sillimanite Chemistry & Crystallography

Chemical Name

aluminum silicate

Chemical Formula

Al2SiO6

Synthesis

Crystal System

Orthorhombic

Classification

Silicate

Nature

Natural

Crystallinity

Crystalline

Comments

Sillimanite Characteristic Physical Properties

Hardness

6-7.5

Streak

White

Specific Gravity

3.14-3.27 Typical:3.25

Toughness

Poor

Inclusions

Sillimanite is often vertically striated and they might contain rutile needles or hypersthene's fibers.

Luster

Vitreous

Stability

Fracture

Uneven

Cleavage

Perfect, in one direction

Comments

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