How to identify Moonstone

To correctly identify Moonstone (Adularia (a variety of Orthoclase Feldspar)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Colorless, white, gray, brown, yellow, orange, green, pink. The most prized moonstones exhibit a bluish-white adularescence.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly, with a characteristic adularescence.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically massive, sometimes in tabular or prismatic crystals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic, often found as anhedral to subhedral masses or as blocky to tabular crystals. Adularia, a common form, often occurs in pseudo-orthorhombic crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, intersecting at approximately 90 degrees (90° 0' 0'').

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Moonstone (adularia variety of orthoclase) forms in low-temperature hydrothermal veins, pegmatites, and alpine-type fissures. It is also found in metamorphic rocks and as a detrital mineral in sedimentary deposits.

Key Facts

Hardness

6-6.5 on the Mohs scale

Specific Gravity

2.56-2.59

Crystal System

Monoclinic

Color

Colorless, white, gray, brown, yellow, orange, green, pink; with bluish-white adularescence.

Luster

Vitreous to pearly

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions (001) and (010) at approximately 90 degrees.

Composition

KAlSi3O8 (Potassium Aluminum Silicate)

Physical characteristics

• Crystal Habit: Massive, granular, tabular, prismatic, often twinned (Carlsbad, Baveno, Manebach twins). Adularia often forms pseudo-orthorhombic crystals.
• Cleavage Type: Perfect in two directions, intersecting at 90 degrees.
• Fracture Type: Uneven to conchoidal
• Tenacity: Brittle
• Luster Type: Vitreous to pearly, with adularescence

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