How to identify Moonstone

To correctly identify Moonstone (Adularia or Orthoclase (feldspar group)), 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, peach, pink, green, or brown. The most desirable moonstones exhibit a blue adularescence.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth when polished (cabochon cut). In its raw form, it can be somewhat uneven.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic, typically found as prismatic or tabular crystals, often twinned. However, gem-quality moonstone is usually found as massive or granular aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, intersecting at approximately 90 degrees (90° 30').

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in igneous rocks (such as granites, syenites, and pegmatites), metamorphic rocks (like gneisses), and hydrothermal veins. It is also found in alluvial deposits due to its relative hardness and resistance to weathering.

Key Facts

Hardness

6 - 6.5 on the Mohs scale.

Specific Gravity

2.56 - 2.59.

Crystal System

Monoclinic.

Color

Colorless, white, gray, peach, pink, green, brown; often with a blue, white, or rainbow adularescence.

Luster

Vitreous to pearly.

Transparency

Transparent to translucent.

Fracture

Uneven to conchoidal.

Cleavage

Perfect in two directions, intersecting at approximately 90 degrees (90° 30').

Composition

Potassium aluminum silicate (KAlSi3O8), with intergrown albite (NaAlSi3O8) layers.

Physical characteristics

• Crystal Habit: Prismatic, tabular, massive, granular. Often twinned (Carlsbad, Baveno, Manebach twins).
• Cleavage Type: Perfect on {001} and {010}.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous on crystal faces, pearly on cleavage surfaces.

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