How to identify Sunstone

To correctly identify Sunstone (Oligoclase (Plagioclase 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 yellow, orange, pink, red, or green, often with a metallic glitter. The color is often unevenly distributed.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to sub-vitreous, with a distinct metallic glitter (aventurescence) from inclusions.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly granular, depending on crystal size and presence of inclusions. Often exhibits a shimmering internal play of light.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually found as anhedral to subhedral grains within igneous rocks. When well-formed, it can occur as tabular or blocky crystals, though gem-quality material is often massive or in irregular fragments.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, nearly at 90 degrees (86° and 94°), characteristic of plagioclase feldspars.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Primarily in mafic igneous rocks (e.g., basalt flows, gabbro), but also in some felsic igneous rocks (e.g., granite, syenite) and occasionally in metamorphic rocks.

Key Facts

Hardness

6 to 6.5 on the Mohs scale

Specific Gravity

2.62 to 2.67

Crystal System

Triclinic

Color

Colorless, yellow, orange, pink, red, green; often with metallic glitter (aventurescence).

Luster

Vitreous to sub-vitreous, with aventurescence.

Transparency

Transparent to translucent.

Fracture

Uneven to conchoidal.

Cleavage

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

Composition

(Na,Ca)(Al,Si)AlSi2O8 - a solid solution of albite and anorthite, specifically oligoclase (An10-An30).

Physical characteristics

• Crystal Habit: Typically anhedral to subhedral grains; rarely well-formed tabular or blocky crystals. Often massive or granular.
• Cleavage Type: Perfect on {001} and {010}, intersecting at approximately 86°.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to sub-vitreous, with a characteristic metallic glitter (aventurescence) due to inclusions.

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