How to identify Amethyst Geode with Stilbite

To correctly identify Amethyst Geode with Stilbite (Quartz (SiO2) and Stilbite-Ca ((Na,Ca)9[Al9Si27O72]·nH2O)), 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

   Amethyst: Purple (ranging from pale lilac to deep violet). Stilbite: White, cream, salmon-pink, orange, or brownish-red. Geode exterior: Typically dull, often dark gray to black basalt or brownish sedimentary rock.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Amethyst: Vitreous (glassy). Stilbite: Vitreous to pearly on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Amethyst: Smooth, glassy crystal faces. Stilbite: Often delicate, radiating, or sheaf-like aggregates, sometimes with a slightly rough or granular feel due to crystal habit. Geode exterior: Rough, often vesicular.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amethyst: Hexagonal prisms terminated by rhombohedral faces, often forming drusy linings. Stilbite: Tabular crystals, often aggregated into characteristic sheaf-like, bow-tie, or radiating forms. Individual crystals are monoclinic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Amethyst (Quartz): None (conchoidal fracture). Stilbite: Perfect on {010}, good on {100}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic environments, particularly in vesicles within basaltic lava flows, or less commonly in hydrothermal veins within sedimentary or metamorphic rocks. The presence of both amethyst and stilbite indicates a multi-stage hydrothermal deposition process.

Key Facts

Hardness

Amethyst (Quartz): 7 on Mohs scale. Stilbite: 3.5-4 on Mohs scale.

Specific Gravity

Amethyst (Quartz): 2.65 g/cm³. Stilbite: 2.12-2.22 g/cm³.

Crystal System

Amethyst (Quartz): Trigonal. Stilbite: Monoclinic.

Color

Amethyst: Purple. Stilbite: White, cream, salmon-pink, orange, brownish-red.

Luster

Amethyst: Vitreous. Stilbite: Vitreous to pearly.

Transparency

Amethyst: Transparent to translucent. Stilbite: Transparent to translucent.

Fracture

Amethyst (Quartz): Conchoidal. Stilbite: Uneven to subconchoidal.

Cleavage

Amethyst (Quartz): None. Stilbite: Perfect on {010}, good on {100}.

Composition

Amethyst (Quartz): Silicon dioxide (SiO2). Stilbite-Ca: Hydrated sodium calcium aluminum silicate ((Na,Ca)9[Al9Si27O72]·nH2O).

Physical characteristics

• Crystal Habit: Amethyst: Prismatic, often terminated, forming drusy linings. Stilbite: Tabular, often aggregated into sheaf-like, bow-tie, or radiating forms.
• Cleavage Type: Amethyst (Quartz): Absent. Stilbite: Perfect basal cleavage {010}, good {100}.
• Fracture Type: Amethyst (Quartz): Conchoidal. Stilbite: Uneven to subconchoidal.
• Tenacity: Amethyst (Quartz): Brittle. Stilbite: Brittle.
• Luster Type: Amethyst (Quartz): Vitreous. Stilbite: Vitreous to pearly.

Explore Amethyst Geode with Stilbite