Ruby is a crystal and a precious gemstone. It is a naturally occurring solid that adheres to the scientific criteria defining crystalline materials. Ruby is categorized as a gem variety of the mineral corundum, which forms deep within the Earth under intense heat and pressure. Its crystalline nature grants it remarkable physical durability and distinct optical properties.
What Defines a Crystal?
A crystal is defined by a highly organized arrangement of its constituent atoms, ions, or molecules. This specific organization is known as a crystal lattice, a three-dimensional pattern that repeats uniformly throughout the entire structure. This characteristic, called long-range order, separates crystals from amorphous solids, such as glass, which have a random atomic structure.
The repeating geometry of the crystal lattice causes crystals to exhibit anisotropy. This means that physical properties, such as hardness or the speed of light passing through the material, have different values when measured along different directions. This directional dependence contrasts with amorphous materials, which are isotropic, meaning their properties are the same in all directions.
The Chemical Makeup of Ruby
Ruby is the red gem variety of the mineral corundum. The chemical composition of corundum is aluminum oxide, represented by the chemical formula Al₂O₃. Pure corundum is colorless, composed only of aluminum and oxygen atoms locked together in their specific structural arrangement.
The brilliant red color of ruby is due to trace elements incorporated during its formation. Specifically, a small amount of chromium replaces some of the aluminum atoms within the crystal lattice. This substitution of chromium for aluminum is responsible for the intense red hue.
If a corundum specimen contains a different trace element, such as iron or titanium, it will display a different color and be classified as a sapphire. The presence of chromium distinguishes red corundum from all other colors of corundum. Although the concentration of this trace element is very low, it dramatically alters the stone’s interaction with light.
Ruby’s Atomic Architecture
The crystalline architecture of ruby, which is the same as all corundum, belongs to the trigonal crystal system. This structure is built upon a dense packing of oxygen atoms arranged in a slightly distorted hexagonal close packing. Aluminum ions precisely fit into two-thirds of the available octahedral spaces between these oxygen layers.
This specific arrangement of aluminum and oxygen atoms forms the crystal lattice that extends indefinitely in three dimensions. The strong atomic bonds within this structure are why ruby is exceptionally hard, rating a 9 on the Mohs scale of mineral hardness, second only to diamond.
The trace chromium atoms substitute for aluminum ions without disrupting the overall crystalline geometry. This substitution maintains the long-range order of the corundum lattice, confirming its status as a single crystal. The directional nature of its atomic bonds also explains why ruby exhibits pleochroism, where the stone appears to show slightly different colors when viewed from various angles.