How to identify Moonstone with Black Tourmaline

To correctly identify Moonstone with Black Tourmaline (Orthoclase Feldspar with Schorl), 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

   Moonstone: typically colorless, white, grey, or pale blue, exhibiting adularescence (a bluish-white sheen). Black Tourmaline: opaque black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Moonstone: vitreous to pearly on cleavage surfaces. Black Tourmaline: vitreous to resinous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Moonstone: smooth, often with a characteristic internal glow. Black Tourmaline: typically prismatic, often with striations parallel to the c-axis, can be rough or fibrous.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Moonstone: typically anhedral to subhedral masses, sometimes tabular or blocky crystals. Black Tourmaline: typically euhedral to subhedral prismatic crystals, often with a triangular cross-section.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Moonstone: perfect in two directions at nearly 90 degrees (orthoclase cleavage). Black Tourmaline: indistinct to poor cleavage, conchoidal to uneven fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Granitic pegmatites, granites, and hydrothermal veins. The presence of both minerals together strongly points to these environments.

Key Facts

Hardness

Moonstone: 6-6.5 (Mohs); Black Tourmaline: 7-7.5 (Mohs).

Specific Gravity

Moonstone: 2.56-2.59; Black Tourmaline: 3.06-3.26.

Crystal System

Moonstone: Monoclinic; Black Tourmaline: Trigonal.

Color

Moonstone: Colorless, white, grey, pale blue with adularescence. Black Tourmaline: Opaque black.

Luster

Moonstone: Vitreous to pearly. Black Tourmaline: Vitreous to resinous.

Transparency

Moonstone: Translucent to transparent. Black Tourmaline: Opaque.

Fracture

Moonstone: Uneven to conchoidal. Black Tourmaline: Conchoidal to uneven.

Cleavage

Moonstone: Perfect in two directions at nearly 90 degrees. Black Tourmaline: Indistinct to poor.

Composition

Moonstone: Potassium aluminum silicate (KAlSi3O8). Black Tourmaline: Complex borosilicate of sodium, iron, and aluminum (Na(Fe2+3)Al6(BO3)3Si6O18(OH)4).

Physical characteristics

• Crystal Habit: Moonstone: Anhedral to subhedral masses, tabular or blocky crystals. Black Tourmaline: Prismatic, often acicular or columnar, with characteristic triangular cross-section and striations.
• Cleavage Type: Moonstone: Perfect {001} and {010} at approximately 90 degrees. Black Tourmaline: Indistinct {1011} and {0001}.
• Fracture Type: Moonstone: Uneven to conchoidal. Black Tourmaline: Conchoidal to uneven.
• Tenacity: Moonstone: Brittle. Black Tourmaline: Brittle.
• Luster Type: Moonstone: Vitreous to pearly. Black Tourmaline: Vitreous to resinous.

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