How to identify Moonstone

To correctly identify Moonstone (Orthoclase feldspar (variety adularia)), 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, gray, peach, yellow, or blue, often with a characteristic bluish-white sheen (adularescence).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to the touch, often cut and polished into cabochons.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic system. Crystals are typically prismatic, often twinned (Carlsbad, Baveno, Manebach twins). In moonstone, it often occurs as massive or anhedral grains within veins or rock matrices.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect cleavage in two directions, intersecting at approximately 90 degrees (specifically, 90° 0' 0" for orthoclase).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Low-temperature hydrothermal veins, alpine-type fissures, pegmatites, and some metamorphic rocks. It is often found in association with quartz, albite, and other feldspars.

Key Facts

Hardness

6-6.5 on the Mohs scale.

Specific Gravity

2.56-2.59 g/cm³.

Crystal System

Monoclinic.

Color

Colorless, white, gray, peach, yellow, blue; often with a bluish-white adularescence.

Luster

Vitreous to pearly.

Transparency

Transparent to translucent.

Fracture

Uneven to conchoidal.

Cleavage

Perfect in two directions, intersecting at approximately 90 degrees (specifically {001} and {010}).

Composition

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

Physical characteristics

• Crystal Habit: Prismatic, tabular, often twinned (Carlsbad, Baveno, Manebach). Also massive, granular, or lamellar.
• Cleavage Type: Perfect on {001} and {010}, intersecting at 90°.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly.

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