How to identify Moonstone

To correctly identify Moonstone (Adularia (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, white, gray, peach, or brown body color, with a characteristic bluish-white to silvery sheen (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, often polished when cut as a gemstone. Can appear somewhat translucent to opaque.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Adularia (orthoclase moonstone) typically forms in prismatic or tabular crystals, often twinned. Plagioclase moonstones can also be tabular. Often found as massive or granular aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect cleavage in two directions at nearly 90 degrees (orthoclase) or slightly less than 90 degrees (plagioclase).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Adularia moonstone is found in low-temperature hydrothermal veins, alpine-type fissures, and some pegmatites. Plagioclase moonstone (peristerite) is found in pegmatites and other igneous rocks.

Key Facts

Hardness

6-6.5 on the Mohs scale.

Specific Gravity

2.56-2.59 (Orthoclase); 2.61-2.62 (Plagioclase).

Crystal System

Monoclinic (Orthoclase/Adularia); Triclinic (Plagioclase).

Color

Colorless, white, gray, peach, brown, with a characteristic bluish-white to silvery adularescence.

Luster

Vitreous to pearly.

Transparency

Transparent to translucent.

Fracture

Uneven to conchoidal.

Cleavage

Perfect in two directions, nearly at 90 degrees (orthoclase) or slightly less (plagioclase).

Composition

Potassium aluminum silicate (KAlSi3O8) for orthoclase/adularia moonstone; Sodium calcium aluminum silicate ((Na,Ca)(Al,Si)4O8) for plagioclase moonstone.

Physical characteristics

• Crystal Habit: Prismatic, tabular, massive, granular.
• Cleavage Type: Perfect in two directions, nearly perpendicular.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly.

Explore Moonstone