How to identify Moonstone

To correctly identify Moonstone (Orthoclase Feldspar (Adularia 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, but can also be gray, peach, yellow, or blue. The most distinctive feature is the adularescence, a milky or bluish sheen that appears to float just beneath the surface.

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 rough form, it can be massive or show distinct crystal faces.

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 often found as massive aggregates. Adularia often forms pseudo-orthorhombic crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Hydrothermal veins, pegmatites, granites, syenites, and metamorphic rocks. The adularia variety is common in alpine-type fissures and low-temperature hydrothermal deposits. Plagioclase moonstone is found in more diverse igneous and metamorphic settings.

Key Facts

Hardness

6-6.5 on the Mohs scale.

Specific Gravity

2.56-2.59 (Orthoclase Moonstone); 2.56-2.62 (Plagioclase Moonstone).

Crystal System

Monoclinic (Orthoclase/Adularia); Triclinic (Plagioclase/Peristerite).

Color

Colorless, white, gray, peach, yellow, blue; characterized by 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 moonstone; Sodium calcium aluminum silicate ((Na,Ca)Al(Si,Al)Si2O8) for plagioclase moonstone.

Physical characteristics

• Crystal Habit: Prismatic, tabular, blocky, or massive. Adularia often forms pseudo-orthorhombic crystals.
• Cleavage Type: Perfect in two directions, {001} and {010}.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly.

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