How to identify Rainbow Moonstone

To correctly identify Rainbow Moonstone (Orthoclase Feldspar (variety adularia) with plagioclase inclusions), 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 or light gray, with a characteristic blue to multicolored adularescence (schiller effect).

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 to slightly uneven fracture surfaces. Often cut and polished into cabochons.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually found as anhedral to subhedral masses or cleavable fragments. Crystals, when present, are typically tabular or prismatic, often twinned.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, nearly at 90 degrees (typically 90° 30').

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in pegmatites, hydrothermal veins, and some igneous and metamorphic rocks, particularly those rich in alkali feldspars. Often associated with granitic and syenitic intrusions.

Key Facts

Hardness

6 - 6.5 on the Mohs scale.

Specific Gravity

2.56 - 2.59 (for orthoclase), 2.68 - 2.72 (for labradorite, which is often the actual mineral for 'Rainbow Moonstone').

Crystal System

Monoclinic (for orthoclase/adularia), Triclinic (for plagioclase/labradorite). The intergrowth creates a complex structure.

Color

Colorless, white, gray, with blue, green, yellow, or rainbow adularescence.

Luster

Vitreous to pearly.

Transparency

Translucent to semi-transparent.

Fracture

Uneven to conchoidal.

Cleavage

Perfect in two directions, nearly at 90 degrees (typically {001} and {010}).

Composition

Potassium Aluminum Silicate (KAlSi3O8) for orthoclase, with Sodium Calcium Aluminum Silicate ((Na,Ca)Al(Si,Al)Si2O8) for plagioclase inclusions.

Physical characteristics

• Crystal Habit: Massive, granular, tabular, prismatic. Often found as cleavable masses.
• Cleavage Type: Perfect in two directions, nearly at 90 degrees.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly.

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