How to identify Amethyst and Fluorite

To correctly identify Amethyst and Fluorite (Quartz (Amethyst variety) and Fluorite), 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: Violet to deep purple. Fluorite: Wide range including purple, green, blue, yellow, pink, brown, colorless; often banded or zoned. The combination will show both colors.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Amethyst: Vitreous (glassy). Fluorite: Vitreous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Amethyst: Typically crystalline, often forming euhedral crystals. Fluorite: Crystalline, often forming well-defined cubic, octahedral, or dodecahedral crystals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amethyst: Hexagonal prisms terminating in rhombohedra. Fluorite: Cubic, octahedral, or dodecahedral crystals; also massive, granular, or botryoidal habits.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Amethyst: None (conchoidal fracture). Fluorite: Perfect octahedral cleavage in four directions.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Hydrothermal veins, vugs, and geodes within igneous (e.g., granites, basalts) and metamorphic rocks, or as gangue minerals in ore deposits. Also found in some sedimentary environments.

Key Facts

Hardness

Amethyst: 7 (Mohs). Fluorite: 4 (Mohs).

Specific Gravity

Amethyst: 2.65 g/cm³. Fluorite: 3.18 g/cm³.

Crystal System

Amethyst: Trigonal. Fluorite: Isometric (Cubic).

Color

Amethyst: Violet to deep purple. Fluorite: Colorless, white, purple, blue, green, yellow, pink, red, brown, black; often zoned.

Luster

Vitreous (glassy) for both.

Transparency

Amethyst: Transparent to translucent. Fluorite: Transparent to translucent.

Fracture

Amethyst: Conchoidal. Fluorite: Subconchoidal to uneven.

Cleavage

Amethyst: None. Fluorite: Perfect octahedral (4 directions).

Composition

Amethyst: Silicon dioxide (SiO2). Fluorite: Calcium fluoride (CaF2).

Physical characteristics

• Crystal Habit: Amethyst: Prismatic, often terminated, massive. Fluorite: Cubic, octahedral, dodecahedral, massive, granular, botryoidal.
• Cleavage Type: Amethyst: None. Fluorite: Perfect octahedral {111}.
• Fracture Type: Amethyst: Conchoidal. Fluorite: Subconchoidal to uneven.
• Tenacity: Amethyst: Brittle. Fluorite: Brittle.
• Luster Type: Vitreous for both.

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