How to identify Black Moonstone

To correctly identify Black Moonstone (Orthoclase Feldspar (variety)), 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

   Dark grey, black, brownish-black body color, often with a silvery, bluish, or brownish adularescence (schiller).

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, sometimes in tabular or prismatic crystals. The adularescence is a key textural/optical feature.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic system, typically forming tabular or prismatic crystals, though often found in massive or anhedral forms in Black Moonstone varieties. Crystals can be twinned (e.g., Carlsbad, Baveno, Manebach twins).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect cleavage in two directions, nearly at right angles (90 degrees).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in igneous rocks (e.g., granites, syenites, pegmatites) and some metamorphic rocks (e.g., gneisses). The specific conditions for the formation of the dark inclusions are varied but generally involve the presence of iron-rich fluids or minerals during crystallization or subsequent alteration.

Key Facts

Hardness

6 - 6.5 (Mohs scale)

Specific Gravity

2.56 - 2.59

Crystal System

Monoclinic

Color

Dark grey, black, brownish-black with adularescence

Luster

Vitreous to pearly

Transparency

Translucent to opaque

Fracture

Uneven to conchoidal

Cleavage

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

Composition

Potassium aluminum silicate (KAlSi3O8) with fine mineral inclusions

Physical characteristics

• Crystal Habit: Typically massive, anhedral grains, or tabular to prismatic crystals; often twinned.
• Cleavage Type: Perfect in two directions, nearly at right angles (90 degrees).
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly on cleavage surfaces.

Explore Black Moonstone