The Crystal Structure of Rubies in Chemistry

Basic Chemical Properties

• Rubies are composed of oxygen and aluminum atoms arranged in a crystal lattice, in a ratio of roughly two aluminum atoms for every three oxygen atoms. Some of these aluminum atoms can be replaced by chromium atoms. Because aluminum technically yields electrons to oxygen, the bonding in these crystals is ionic; however, the ions formed pack so closely together that the outer shells of the atoms where the electrons reside fuse to some degree. This is what makes the structure of a ruby crystal incredibly strong and dense.

Crystal Family

• The crystals in ruby are in the hexagonal class of crystals, giving them four axes of symmetry -- one vertical and three in one plane. It is this plane that gives the class its "hexagonal" name since the three axes are spaced evenly, creating a hexagonal shape. Corundum crystals fall into the subcategory that has lower symmetry, known as trigonal, only having three-fold symmetry around the vertical axis. A number of other well-known minerals are in this same class, such as beryl. Both aquamarine and emerald are beryls.

Specific Crystallization Patterns

• Officially, rubies display a particular crystal structure known as the hexagonal scalenohedron. A scalenohedron is a twelve-sided polyhedron, and each of its faces is an identical scalene triangle, or a triangle with three different side lengths. These faces are arranged with six triangles, each forming two pyramid-like shapes, albeit with uneven bottom edges. These two pyramidal structures meet along their open edges. This structure as a whole is known in geometry as a hexagonal scalenohedron and so gives its name to the crystallization pattern of corundum.

Crystalline Complications

• The hexagonal scalenohedron crystal pattern is one of the mineral patterns that most often displays irregularities of form. This is where, instead of forming perfectly regular structures composed entirely of scalenohedrons, the ions will "slip" into other, similar crystalline shapes for short stretches of the mineral. These discontinuities are known as "form combinations" and give naturally-formed rubies the fascinating variety of shapes and faces they display. Other related minerals of the "hematite group" with similar metal oxide compositions display similar rates of form combinations.