A diamond is classified simultaneously by chemists, geologists, and gemologists. It is widely recognized as a substance of extreme durability and exceptional optical properties, making it highly valued in both industrial and luxury markets. Understanding what a diamond is requires breaking down its identity across different scientific disciplines and the commercial standards used to determine its worth. This classification system moves from its chemical makeup and atomic architecture to the criteria used for its valuation as a gemstone.
Elemental Composition
The most fundamental classification of a diamond is as an elemental solid, meaning its composition is almost entirely a single chemical element: carbon. This makes a diamond a form of pure, crystallized carbon. Chemically, the diamond is defined as an allotrope of carbon, which is the term for different structural forms of the same element.
The physical properties of an allotrope are determined by the arrangement of its atoms, not the atoms themselves. For example, graphite is also an allotrope of carbon, yet it has dramatically different characteristics from the diamond. This contrast illustrates that a diamond’s classification is defined by more than just its chemical formula of “C”.
Mineral Identity
From a geological perspective, the diamond is classified as a mineral. To be recognized as a mineral, a substance must meet four criteria: it must be naturally occurring, inorganic, a solid, and possess a definite chemical composition with an ordered atomic arrangement. The diamond meets these requirements by forming naturally deep within the Earth’s mantle under heat and pressure.
Since it is not derived from living organisms, the diamond is considered inorganic. Its consistent chemical makeup of pure carbon gives it a definite composition, and its highly organized internal structure fulfills the final requirement for mineral classification. The diamond belongs to the native element group of minerals, a collection of substances composed of a single element, such as gold or sulfur.
Crystalline Structure
A diamond’s physical properties are a direct result of its internal atomic arrangement, known as its crystal structure. Each carbon atom is linked to four neighboring carbon atoms, forming a repeating tetrahedral structure. These connections are strong covalent bonds, creating an extremely rigid, three-dimensional lattice known as the diamond cubic crystal system.
This dense and strong arrangement gives the diamond its unparalleled hardness. It ranks as a perfect 10 on the Mohs scale of hardness, classifying it as the hardest known natural material. In contrast, graphite’s carbon atoms are arranged in weak, sliding layers, which is why it is soft and used as a lubricant.
Commercial Grading Criteria
Beyond its scientific descriptions, the diamond is classified commercially through a standardized system used primarily for gem-quality stones. This universal method, established by the Gemological Institute of America (GIA), is known as the “4 Cs,” and it determines a stone’s aesthetic quality and market value.
The 4 Cs
The first C, Carat, classifies the stone by its physical weight; one metric carat is defined as 200 milligrams. The second C is Cut, which evaluates the craftsmanship of the diamond’s proportions, symmetry, and polish, determining how effectively it interacts with light. A high-quality cut maximizes a diamond’s brilliance, fire, and scintillation.
Clarity, the third C, classifies the stone based on the presence of internal features called inclusions and external imperfections called blemishes. Stones are graded on a scale ranging from Flawless (FL) to Included (I), with higher clarity grades indicating fewer imperfections visible under 10x magnification.
The final C is Color, which refers to the degree of colorlessness in a white diamond, graded on a scale from D (colorless) to Z (light yellow or brown). Colorless diamonds are the most rare and highly valued in this classification. These four categories provide a precise, objective language for classifying and valuing a diamond in the global jewelry trade.