Moonstone | Gemopedia

Scientifically speaking, moonstone is the adularescent variety of orthoclase feldspar, but that definition falls short in describing this enchanting stone. When looking at a moonstone that glows like a bright full moon, it's easy to understand how the milky stones got their name. In his records of natural history, Pliny the Elder wrote about star stones he called astrions, believed to be what we now know as star moonstones, or moonstones that display asterism. Pliny described an astrion as a stone that imprisons a bright star, and although...it is like rock crystal, [it] has a brilliant blue sheen and radiating from the centre shines a star with the full brilliancy of the moon.
Alternate
Names
Star Moonstone

Moonstone Classification

Common Name

Moonstone

Species

Orthoclase

Variety

Moonstone

Colors

colorless to white, orange, yellow or brown

Alternate Names

star moonstone

Gemstone Groups

Group: Feldspar

Key Separations

RI, biaxial interference figure in polariscope and magnification. Fluorescence and SG might also be useful.

Comments

Moonstone can also be labradorite feldspar.

Moonstone Optical Properties

Transparency

Semitransparent To Opaque

Refractive Index

1.518-1.526 Tolerance: (+0.010/-0.010)

Birefringence

0.005-0.008

Optic Character

biaxial

Optic Sign

negative

Polariscope Reaction

doubly refractive (DR) or AGG depending on transparency

Fluorescence

SWUV: orange; maybe pink to red
LWUV: inert to blue; maybe pink to red

CCF Reaction

Pleochroism

Value: None in colorlessYellow material colorless and light yellow

Dispersion

Moonstone Chemistry & Crystallography

Chemical Name

potassium aluminum silicate

Chemical Formula

KAlSi3O8

Synthesis

Crystal System

Monoclinic

Classification

Silicate

Nature

Natural

Crystallinity

Polycrystalline

Moonstone Characteristic Physical Properties

Hardness

6

Streak

white

Specific Gravity

2.55-2.61 Range: +/-0.03 Typical: 2.58

Toughness

Poor

Inclusions

Moonstone will have blue or white adularescence. Stones might have centipede like inclusions and material with aventuresence or asterism might contain reflective inclusions such as fine needles or elongated hematite platelets.

Luster

Vitreous

Stability

Good

Fracture

Uneven To Splintery

Cleavage

Good In Two Directions: Parting Common

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