How to identify Moonstone

To correctly identify Moonstone (Adularia (Orthoclase variety) or Albite (Plagioclase Feldspar variety)), 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

   Typically colorless to white, but can also be gray, brown, pink, orange, green, or yellow. The body color is often translucent to semi-transparent.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly, especially on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly uneven, depending on the crystal habit and presence of cleavage planes.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic (orthoclase variety) or Triclinic (albite variety). Often found as massive, granular, or in tabular to prismatic crystals. Gem-quality material is usually massive or in cleavable masses.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, intersecting at or near 90 degrees (orthoclase) or slightly less than 90 degrees (albite). This is a key characteristic of feldspar minerals.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in pegmatites, syenites, and other igneous rocks, as well as in hydrothermal veins and some metamorphic rocks. It is often associated with late-stage magmatic differentiation.

Key Facts

Hardness

6 - 6.5 on the Mohs scale.

Specific Gravity

2.56 - 2.59 (Orthoclase Moonstone); 2.57 - 2.75 (Albite Moonstone).

Crystal System

Monoclinic (Orthoclase variety); Triclinic (Albite variety).

Color

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

Luster

Vitreous to pearly.

Transparency

Transparent to translucent.

Fracture

Uneven to conchoidal.

Cleavage

Perfect in two directions, intersecting at approximately 90 degrees (orthoclase) or slightly less (albite).

Composition

Potassium Aluminum Silicate (KAlSi3O8) for orthoclase moonstone; Sodium Aluminum Silicate (NaAlSi3O8) for albite moonstone.

Physical characteristics

• Crystal Habit: Massive, granular, tabular to prismatic crystals. Often found as cleavable masses.
• Cleavage Type: Perfect in two directions, {001} and {010}, intersecting at 90 degrees (orthoclase) or near 90 degrees (albite).
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly.

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