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Elemental Composition
The chemical formula of emeralds is Be3Al2(SiO3)6. In other words, they are composed of three beryllium atoms, two aluminum atoms, and six silicon trioxide molecules. They are classified in the beryl group.
Crystal Properties
Emeralds are placed in the crystal class of cyclosilicates. They have a hexagonal crystal system, which refers to the shape of the crystal lattices that make up their structure. Their crystal habit, according to Mineral Miners, involves "typically small elongate first order prisms, often with vertical striations and irregular pinacoidal terminations."
Visual Properties
Emeralds have a vitreous, or glassy luster. Their transparency ranges from translucent to opaque; in rare cases, they are completely transparent. Emeralds fracture conchoidally, meaning they split into smooth, shiny, slightly brittle surfaces. Their pleochroism, which refers to the quality of having apparently different colors depending on the angle with which one looks, is classified as "distinct," varying from blue-green to yellow-green.
Quantitative Properties
Emeralds have a specific gravity, which is the ratio of its density to the density of water, of between 2.67 and 2.78. They possess a birefringence of between .005 and .008 and a refraction index of between 1.566 and 1.602. Birefringence refers to the splitting of light into two rays when it passes through a crystal. The refraction index measures how quickly light travels through the emerald. The emerald's hardness scale ranges from 7.5 to 8.0. Hardness scale is a factor of the degree to which a substance can be visibly scratched by another substance.