How to identify Moonstone

To correctly identify Moonstone (Adularia (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

   Typically colorless to white, but can also be gray, brown, yellow, pink, or green. 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, often exhibiting a characteristic 'schiller' or 'adularescence' effect.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually found in massive or tabular forms. Crystals are typically monoclinic, often prismatic or blocky, but well-formed crystals are rare in gem quality material.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, intersecting at approximately 90 degrees (specifically 90 degrees and 90 degrees 30 minutes).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Occurs in pegmatites, granites, syenites, and some metamorphic rocks. It is often found in hydrothermal veins and alpine-type fissures, particularly the adularia variety.

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, pink, green; with blue or white adularescence.

Luster

Vitreous to pearly.

Transparency

Transparent to translucent.

Fracture

Uneven to conchoidal.

Cleavage

Perfect in two directions (basal {001} and prismatic {010}) at approximately 90 degrees.

Composition

Potassium aluminum silicate (KAlSi3O8).

Physical characteristics

• Crystal Habit: Typically massive, tabular, or prismatic. Often found as cleavage fragments or anhedral grains in rocks. Well-formed crystals are rare in gem quality.
• Cleavage Type: Perfect in two directions, intersecting at 90 degrees and 90 degrees 30 minutes.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly on cleavage surfaces.

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