How to identify Rainbow Moonstone

To correctly identify Rainbow Moonstone (Moonstone (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 white to colorless, translucent to semi-transparent, with a characteristic iridescent play of colors (blue, green, yellow, etc.) when light interacts with its internal structure.

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). Unpolished surfaces may show typical feldspar cleavage planes.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually found as anhedral to subhedral masses or cleavable fragments. When euhedral, it forms tabular or prismatic crystals, though these are rare in gem quality.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, nearly at right angles (approximately 90 degrees for orthoclase, 86 degrees for plagioclase).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in various igneous rocks (e.g., anorthosites, gabbros, basalts, syenites, granites, pegmatites) and some metamorphic rocks. It can also occur in hydrothermal veins.

Key Facts

Hardness

6 - 6.5 (Mohs scale)

Specific Gravity

2.56 - 2.62

Crystal System

Monoclinic (for orthoclase moonstone) or Triclinic (for plagioclase/labradorite rainbow moonstone)

Color

Colorless to white, with adularescence/labradorescence displaying blue, green, yellow, and other spectral colors.

Luster

Vitreous to pearly

Transparency

Translucent to semi-transparent

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions, nearly at 90 degrees (orthoclase) or 86 degrees (plagioclase)

Composition

Potassium aluminum silicate (KAlSi3O8) for true moonstone (orthoclase variety) or Sodium Calcium Aluminum Silicate ((Na,Ca)(Al,Si)AlSi2O8) for Rainbow Moonstone (labradorite variety).

Physical characteristics

• Crystal Habit: Massive, granular, lamellar, or tabular to prismatic crystals (rarely euhedral).
• Cleavage Type: Perfect in two directions, {001} and {010}.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly on cleavage surfaces.

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