How to identify Faceted Gemstone

To correctly identify Faceted Gemstone (Unknown Gemstone (likely Amethyst or Iolite based on color)), 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: Shades of purple, from pale lilac to deep violet, sometimes with reddish or bluish secondary hues. Iolite: Typically violet-blue to purplish-blue, often exhibiting strong pleochroism (different colors from different angles).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) for both Amethyst and Iolite.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth and polished due to faceting. Internally, Amethyst can show color zoning or inclusions. Iolite may show inclusions or internal fractures.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amethyst: Typically forms in prismatic hexagonal crystals, often in geodes or drusy coatings. Iolite: Orthorhombic crystals, often short prismatic or tabular, but usually found as granular masses or anhedral grains in metamorphic rocks. Faceted stones are cut from larger crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Amethyst: None (conchoidal fracture). Iolite: Distinct cleavage in one direction ({010}), and indistinct in two others ({100} and {001}).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Amethyst: Hydrothermal veins, vugs, and geodes in volcanic and plutonic igneous rocks, as well as some metamorphic rocks. Iolite: Metamorphic rocks (schists, gneisses, hornfels) formed from aluminum-rich sediments, and occasionally in igneous rocks like granites and pegmatites.

Key Facts

Hardness

Amethyst: 7 on Mohs scale. Iolite: 7-7.5 on Mohs scale.

Specific Gravity

Amethyst: 2.65 g/cm³. Iolite: 2.53-2.78 g/cm³.

Crystal System

Amethyst: Trigonal. Iolite: Orthorhombic.

Color

Purple (Amethyst); Violet-blue, purplish-blue (Iolite, with pleochroism).

Luster

Vitreous.

Transparency

Transparent to translucent.

Fracture

Amethyst: Conchoidal. Iolite: Subconchoidal to uneven.

Cleavage

Amethyst: None. Iolite: Distinct in one direction ({010}), indistinct in two others.

Composition

Amethyst: Silicon dioxide (SiO2). Iolite: Magnesium iron aluminum cyclosilicate ((Mg,Fe)2Al4Si5O18).

Physical characteristics

• Crystal Habit: Amethyst: Prismatic, often terminated, sometimes massive. Iolite: Short prismatic, tabular, or massive granular.
• Cleavage Type: Amethyst: Absent. Iolite: Distinct on {010}, indistinct on {100} and {001}.
• Fracture Type: Amethyst: Conchoidal. Iolite: Subconchoidal to uneven.
• Tenacity: Brittle for both.
• Luster Type: Vitreous.

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