The classification of diamonds has long been a source of confusion because they are valued as a gemstone and often found embedded within large geological formations. Geological science uses a precise set of criteria to categorize Earth’s materials, settling the question definitively. A diamond is classified as a mineral, not a rock, a distinction based on its specific composition and internal structure.
How Geologists Define Minerals and Rocks
Geologists rely on a strict set of requirements to define a substance as a mineral. A mineral must be naturally occurring, meaning it is not created in a laboratory. It must also be a solid under normal surface conditions and generally inorganic, meaning it is not composed of materials derived from living organisms.
The two most fundamental criteria involve internal structure and chemistry. A mineral must possess a definite chemical composition, expressed by a specific chemical formula. Crucially, it must also have an ordered internal atomic arrangement, known as a crystal lattice. This repeating, three-dimensional pattern of atoms determines nearly all of the substance’s physical properties, such as hardness and cleavage.
A rock, by contrast, is defined as a solid aggregate of one or more minerals or mineraloids. Rocks are essentially mixtures, lacking a single, precise chemical formula or a consistent atomic structure throughout the mass. Granite, for example, is a common rock composed of interlocking crystals of several different minerals, typically quartz, feldspar, and mica.
Even a monomineralic rock, such as limestone, which is primarily made of the single mineral calcite, is still considered a rock because it is an aggregate. The limestone mass consists of many individual calcite grains that have been cemented or compacted together. This difference highlights that minerals are the pure, homogeneous building blocks, while rocks are the heterogeneous structures built from them.
The Criteria That Classify Diamond as a Mineral
Diamond satisfies every requirement necessary for classification as a mineral. It is a naturally occurring solid that forms deep within the Earth’s mantle, and it is inorganic. Its chemical composition is simple and definite, consisting of nearly pure carbon atoms, which gives it the chemical formula ‘C’.
The most defining factor is its highly organized internal structure, a perfect example of a crystal lattice. Each carbon atom is covalently bonded to four other carbon atoms in a repeating, three-dimensional tetrahedral arrangement. This specific, rigid structure gives diamond its extraordinary hardness, making it the hardest known natural material on the Mohs scale.
The specific geometry of the diamond’s crystal lattice differentiates it from other carbon-based substances, such as graphite. Graphite’s atoms are arranged in flat sheets that can slide past one another, making it soft enough to use as pencil lead. Diamond’s dense, interlocking structure ensures it meets the criterion for an ordered atomic structure. It is categorized as a native element mineral because it is found in nature in its uncombined elemental state.
Understanding Diamond’s Geological Host Material
The confusion about a diamond being a rock often stems from the fact that diamonds are only found within a larger rock mass. Diamonds form at extreme depths of 150 to 250 kilometers below the surface in the Earth’s upper mantle, requiring intense pressure and high temperatures. They are delivered to the surface through a distinct geological transport mechanism, as they do not form in the Earth’s crust.
This transport occurs via rare, fast, and violent volcanic eruptions that originate in the mantle. These eruptions create vertical, carrot-shaped structures known as volcanic pipes that pierce through the crust. The primary host rocks that fill these pipes and carry the diamonds are Kimberlite and, less commonly, Lamproite.
Kimberlite and Lamproite are both igneous rocks, aggregates of various minerals that solidified from molten magma. Therefore, the diamond is merely an accidental passenger, a mineral crystal trapped and carried upward within the larger rock structure. When a miner extracts diamond-bearing material, they are mining the Kimberlite or Lamproite rock, which holds the diamond mineral within its mass.