How to identify Moonstone

To correctly identify Moonstone (Adularia (Orthoclase variety) or Oligoclase (Plagioclase 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, white, gray, peach, or blue. The body color can be transparent to translucent.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly, especially on cleavage surfaces. The adularescence gives it a unique internal glow.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly uneven, often exhibiting a characteristic internal shimmer.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic (Orthoclase variety) or Triclinic (Oligoclase variety). Crystals are typically prismatic, tabular, or blocky, but moonstone is often found in massive or granular aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, nearly at 90 degrees (orthoclase) or slightly less than 90 degrees (oligoclase).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in pegmatites, granites, syenites, and hydrothermal veins. Also occurs in metamorphic rocks and as detrital grains in sedimentary deposits.

Key Facts

Hardness

6-6.5 (Mohs scale)

Specific Gravity

2.56-2.62 (Orthoclase Moonstone); 2.61-2.69 (Oligoclase Moonstone)

Crystal System

Monoclinic (Orthoclase variety); Triclinic (Oligoclase variety)

Color

Colorless, white, gray, peach, blue; often with adularescence

Luster

Vitreous to pearly

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions, nearly at 90 degrees (orthoclase) or slightly less than 90 degrees (oligoclase)

Composition

Potassium Aluminum Silicate (KAlSi3O8) for Orthoclase Moonstone; Sodium Calcium Aluminum Silicate ((Na,Ca)Al(Si,Al)Si2O8) for Oligoclase Moonstone

Physical characteristics

• Crystal Habit: Prismatic, tabular, blocky; often massive or granular aggregates.
• Cleavage Type: Perfect in two directions, nearly at 90 degrees (orthoclase) or slightly less than 90 degrees (oligoclase).
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly.

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