How to identify Rainbow Moonstone

To correctly identify Rainbow Moonstone (Labradorite (variety of 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

   Body color typically white, colorless, or pale gray to light brown. The defining characteristic is the iridescent play of colors (labradorescence/adularescence) across the surface, which can include blue, green, yellow, orange, pink, and purple.

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

   Typically massive, granular, or in tabular crystals within host rock. When polished, it has a smooth, often slightly waxy feel.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually found as anhedral to subhedral grains in igneous rocks. When well-formed, it can occur as tabular, prismatic crystals, often twinned (polysynthetic twinning visible as fine striations on cleavage surfaces).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, nearly at 90 degrees (86° and 94°), and good in a third direction.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Occurs in mafic igneous rocks such as gabbro, basalt, anorthosite, and norite. Also found in some metamorphic rocks and as detrital grains in sediments derived from these rocks.

Key Facts

Hardness

6 to 6.5 on the Mohs scale.

Specific Gravity

2.68 to 2.72.

Crystal System

Triclinic.

Color

White, colorless, or pale gray body with iridescent flashes of blue, green, yellow, orange, pink, purple (labradorescence/adularescence).

Luster

Vitreous to pearly.

Transparency

Transparent to translucent.

Fracture

Uneven to conchoidal.

Cleavage

Perfect in two directions (pinacoidal), nearly at 90 degrees (86° and 94°).

Composition

(Ca,Na)(Al,Si)AlSi2O8, specifically an intermediate plagioclase feldspar with 50-70% anorthite (CaAl2Si2O8) and 30-50% albite (NaAlSi3O8).

Physical characteristics

• Crystal Habit: Typically massive, granular, or in tabular to prismatic crystals. Often twinned (polysynthetic twinning).
• Cleavage Type: Perfect in two directions, nearly at right angles (86° and 94°).
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly.

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