Lesson7: The Physical and Chemical Properties of Pearls
1. Physical Properties of Pearls
a. Size
Pearl sizes are typically measured by their diameter in millimeters. The size of a pearl significantly impacts its value, with larger pearls being rarer and more prized. International standards classify pearls into several categories:
- Large Pearls: Diameter greater than 8mm
- Medium Pearls: Diameter between 6-8mm
- Small Pearls: Diameter between 5-6mm
- Tiny Pearls: Diameter smaller than 5mm
b. Density
Pearl density varies depending on the composition of the nucleus and ranges between 1.6 to 2.8 g/cm³.
c. Hardness
The abrasion hardness of pearls ranges from 2.5 to 4.5 on the Mohs scale, depending on the type, structure, and formation conditions of the pearl. Compared to most gemstones, pearls are relatively soft and should be stored separately to prevent scratches.
Note: Mohs hardness values for common materials:
- Diamond: Hardness 10 (the hardest naturally occurring substance)
- Corundum: Hardness 9 (rubies and sapphires)
- Topaz: Hardness 8
- Quartz: Hardness 7
- Emerald: Hardness 6-7
- Apatite: Hardness 5
- Fluorite: Hardness 4
- Calcite: Hardness 3
- Pearl: Hardness 2.5-4.5
- Gypsum: Hardness 2
- Talc: Hardness 1
d. Elasticity
Pearls possess a certain degree of elasticity. A high-quality pearl dropped from a height of about 1 meter onto a hard surface (like glass) can bounce to a height of around 40 cm. This elasticity depends on the presence of conchiolin (a protein substance), with higher conchiolin content leading to better elasticity. This property is also used to evaluate pearl quality.
e. Color and Luster
Pearls come in a wide variety of colors, such as white, pink, yellow, and blue hues. The colors are determined by porphyrin and metalloporphyrin compounds (proteins bound to different metal ions). Different metals and their concentrations result in varying colors and shades.
f. Optical Properties
Pearls exhibit unique luster due to their internal microscopic structure, such as nacre layers (thin layers of aragonite). These structures create effects like reflection, refraction, and interference, giving pearls their signature shine.
2. Chemical Composition of Pearls
a. Inorganic Components
The pearl nucleus is primarily composed of calcium carbonate crystals in an aragonite structure. This hexagonal crystal system gives pearls their distinctive luster and hardness.
b. Organic Components
Conchiolin proteins serve as the organic matrix, functioning as a “biological cement” that provides structural integrity. These proteins also bind with pigments to control the pearl’s color and contribute to its elasticity and flexibility.
c. Trace Elements
Pearls contain trace amounts of metal ions, such as manganese, iron, copper, and zinc. Though present in small quantities, these elements play a critical role in the pearl’s color. Additionally, freshwater pearls typically have a higher manganese ion concentration compared to seawater pearls, which is a key factor in distinguishing their origin.