How to identify Moonstone

To correctly identify Moonstone (Adularia (Orthoclase 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, with shades of 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 internal glow.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic (orthoclase variety) or Triclinic (plagioclase variety). Crystals are typically prismatic, tabular, or blocky, but moonstone is usually found in massive or granular forms, often cut as cabochons.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Moonstone (adularia) is typically found in low-temperature hydrothermal veins and alpine-type fissures, often associated with granitic pegmatites, syenites, and other igneous rocks. Peristerite moonstone is found in pegmatites and metamorphic rocks.

Key Facts

Hardness

6.0 - 6.5 on the Mohs scale.

Specific Gravity

2.56 - 2.59 (Orthoclase); 2.55 - 2.75 (Plagioclase).

Crystal System

Monoclinic (Orthoclase variety, Adularia) or Triclinic (Plagioclase variety, Peristerite).

Color

Colorless, white, gray, brown, yellow, pink, green; with characteristic 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 (plagioclase).

Composition

Potassium aluminum silicate (KAlSi3O8) for orthoclase varieties; Sodium calcium aluminum silicate (Na,Ca)(Al,Si)AlSi2O8) for plagioclase varieties.

Physical characteristics

• Crystal Habit: Prismatic, tabular, blocky, massive, granular. Often cut as cabochons to enhance adularescence.
• Cleavage Type: Perfect in two directions, {001} and {010}.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly on cleavage surfaces.

Explore Moonstone