How to identify Moonstone

To correctly identify Moonstone (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 colorless to white, with a characteristic bluish or whitish adularescence. Can also be found in peach, pink, green, or gray hues.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly, especially on cleavage surfaces. The adularescence gives it a unique internal glow.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly uneven on fracture surfaces. Cleavage surfaces are typically smooth.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic system. Crystals are typically prismatic, often tabular, and can be twinned (e.g., Carlsbad twins). However, moonstone is more commonly found as massive or granular aggregates, or as inclusions within other rocks.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect cleavage in two directions, intersecting at approximately 90 degrees (specifically, 90° 0' 0" for orthoclase). This is a key identifying feature of feldspars.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in igneous rocks such as granites, syenites, and especially pegmatites. Also occurs in some metamorphic rocks like gneisses and schists, and in hydrothermal veins. Alluvial deposits are also a significant source due to its relative hardness and resistance to weathering.

Key Facts

Hardness

6-6.5 on the Mohs scale

Specific Gravity

2.56-2.59

Crystal System

Monoclinic

Color

Colorless, white, gray, peach, pink, green; with adularescence (blue, white, rainbow sheen)

Luster

Vitreous to pearly

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions, intersecting at approximately 90 degrees (specifically {001} and {010})

Composition

Potassium aluminum silicate (KAlSi3O8)

Physical characteristics

• Crystal Habit: Prismatic, tabular, massive, granular. Often twinned (Carlsbad, Baveno, Manebach twins).
• Cleavage Type: Perfect on {001} and {010}, intersecting at 90°.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly.

Explore Moonstone