Diamonds are naturally occurring minerals valued for their exceptional hardness, intense brilliance, and enduring beauty. Understanding a diamond requires looking beyond its appearance to its fundamental chemical building blocks and the extreme geological processes that create it. This article details the composition and formation of this unique material.
The Basic Building Block: Pure Carbon
A diamond’s chemical composition is remarkably simple, consisting almost entirely of pure carbon, represented by the single element symbol C. The material is an allotrope of carbon, meaning it is one of several physical forms that carbon can take, much like graphite or charcoal. Unlike those softer, more common forms, a diamond is typically about 99.95 percent carbon, making it a single-element mineral.
The remaining 0.05 percent is often made up of trace elements, which are atoms not part of the diamond’s fundamental chemistry but are incorporated during formation. These impurities are responsible for the wide range of colors seen in natural diamonds. Nitrogen is the most common impurity, often causing yellow or brown tints, while the presence of boron can result in a rare blue color.
The Unique Crystalline Structure
The incredible properties of a diamond, such as its unmatched hardness and high density, do not come from its simple chemical composition but from the precise way the carbon atoms are arranged. Each carbon atom is covalently bonded to four neighboring carbon atoms. This highly uniform arrangement forms a three-dimensional lattice structure known as a tetrahedron.
The resulting covalent bonds are among the strongest known in nature, locking the atoms into an extremely rigid and continuous network. Because the bonding is identical in all directions, the crystal is exceptionally strong and resistant to scratching, giving it the highest rating of 10 on the Mohs scale of mineral hardness. This dense, tightly bonded structure is what differentiates a diamond from other carbon allotropes, such as graphite.
Geological Origin and Formation
Natural diamonds form only under conditions of extremely high pressure and high temperature that exist deep within the Earth’s interior. This formation occurs in the upper mantle, typically at depths ranging from about 90 to 120 miles below the surface. These intense conditions fall within what scientists call the diamond stability field, where carbon atoms can crystallize into the diamond structure rather than the more common graphite structure.
Most diamonds are ancient, having crystallized between one billion and 3.3 billion years ago, which is long before they were brought to the surface. They remain preserved in the mantle until they are transported to the Earth’s crust by rare, deep-source volcanic eruptions. These violent eruptions carry the diamonds upward rapidly, preventing them from converting back into graphite as they pass through regions of lower pressure.
The diamonds are delivered to the surface within carrot-shaped columns of a solidified volcanic rock called kimberlite, or more rarely, lamproite. These structures, known as pipes, are the primary source for almost all mined diamonds.