How to identify Pearl

To correctly identify Pearl (Calcium carbonate (aragonite/calcite) and conchiolin), 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

   Pearls exhibit a wide range of colors, including white, cream, pink, silver, gold, green, blue, and black. The body color is often accompanied by an overtone, a translucent color that appears on the surface, and iridescence, known as orient.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Pearls possess a unique, soft, and deep luster, often described as 'pearly' or 'orient'. This is due to the reflection and refraction of light from the overlapping layers of nacre. High-quality pearls have a sharp, mirror-like reflection.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The surface of a pearl is typically smooth, though some pearls, particularly baroque shapes, may have irregular textures. Under magnification, the surface may show subtle growth lines.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pearls do not exhibit a macroscopic crystal form in the traditional sense, as they are biogenic. Microscopically, they are composed of aragonite (orthorhombic) or calcite (trigonal) crystals arranged in a brick-and-mortar structure with conchiolin.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Pearls do not exhibit cleavage. They have a conchoidal to uneven fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Pearls are found within the soft tissues of marine and freshwater mollusks. Marine pearls are typically found in oysters in warm ocean waters, while freshwater pearls are found in mussels in rivers and lakes. They are not found in geological formations in the same way as minerals, but rather are harvested from living organisms or their shells.

Key Facts

Hardness

2.5-4.5 on the Mohs scale (due to aragonite/calcite and conchiolin). This makes them relatively soft and susceptible to scratching.

Specific Gravity

2.60-2.85 (varies slightly depending on the proportion of aragonite/calcite to conchiolin and the type of pearl).

Crystal System

Orthorhombic (aragonite) or Trigonal (calcite) for the calcium carbonate component. The overall structure is biogenic and amorphous in macroscopic appearance.

Color

Wide range, including white, cream, pink, silver, gold, green, blue, black. Often with iridescent overtones.

Luster

Pearly, iridescent (orient).

Transparency

Opaque to translucent.

Fracture

Conchoidal to uneven.

Cleavage

None.

Composition

Calcium carbonate (CaCO3) in the form of aragonite (predominantly) or calcite, interlayered with an organic protein matrix called conchiolin.

Physical characteristics

• Crystal Habit: Not applicable in the traditional sense for a biogenic material. Microscopically, it consists of tabular aragonite crystals arranged in a 'brick-and-mortar' structure.
• Cleavage Type: None.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Pearly, iridescent.

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