How to identify Moonstone

To correctly identify Moonstone (Adularia (Potassium 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, gray, peach, pink, yellow, or brown, with a characteristic bluish-white to white 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 when polished (cabochon cut). Uneven to conchoidal fracture if broken.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Often found as anhedral to subhedral masses or in tabular to prismatic crystals, though gem-quality material is usually massive and cut into cabochons. Adularia specifically forms pseudo-orthorhombic crystals, often in characteristic 'Adularia habit' (rhombic or tabular).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect cleavage in two directions, intersecting at approximately 90 degrees (characteristic of feldspars).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Moonstone, particularly the adularia variety, is found in low-temperature hydrothermal veins, pegmatites, and alpine-type fissures. It can also occur in some igneous and metamorphic rocks. The formation requires slow cooling to allow for the exsolution of albite and orthoclase lamellae.

Key Facts

Hardness

6-6.5 (Mohs scale)

Specific Gravity

2.56-2.59 g/cm³

Crystal System

Monoclinic (for orthoclase and adularia varieties)

Color

Colorless, white, gray, peach, pink, yellow, brown; with blue or white adularescence.

Luster

Vitreous to pearly.

Transparency

Transparent to translucent.

Fracture

Uneven to conchoidal.

Cleavage

Perfect in two directions, intersecting at approximately 90 degrees.

Composition

Potassium aluminum silicate (KAlSi3O8) with intergrown sodium aluminum silicate (NaAlSi3O8).

Physical characteristics

• Crystal Habit: Typically massive, granular, or in tabular to prismatic crystals (especially adularia).
• Cleavage Type: Perfect basal {001} and good prismatic {010} cleavage, intersecting at 90 degrees.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly on cleavage surfaces.

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