Introduction
The previous article, What Is a Diamond?, explained that diamond is carbon bonded in a three-dimensional sp3 lattice — and that this structure produces its hardness, brilliance, and fire. This article goes deeper into the crystallography: what shape that lattice takes, what forms a diamond crystal grows into, where its planes of weakness lie, and what happens when crystal growth goes slightly wrong.
These are not abstract concerns. The crystal habit of a rough diamond determines how a cutter approaches it. The cleavage planes dictate where a stone can be split cleanly and where it is vulnerable to damage. Twinning affects how light moves through a finished stone. Understanding diamond crystal structure connects the geology of rough diamonds to the decisions that shape every polished stone you will encounter in a jewellery case.
Key Points
The Cubic Crystal System
Minerals are classified into seven crystal systems based on the symmetry of their unit cell — the smallest repeating block that builds up the entire crystal. Diamond belongs to the cubic system, also called the isometric system. In this system, the three crystallographic axes are equal in length and meet at right angles. It is the most symmetrical of all crystal systems.
Diamond's specific structure within the cubic system is a face-centred cubic (FCC) lattice with a two-atom basis. Each carbon atom sits at the centre of a tetrahedron formed by its four nearest neighbours, and the pattern repeats identically in every direction. Because of this high symmetry, diamond is optically isotropic — its refractive index (2.417) is the same regardless of the direction light travels through the crystal. This is a key distinction from many other gemstones. Sapphire (trigonal system) and emerald (hexagonal system) are doubly refractive: light splits into two rays travelling at different speeds depending on direction. In diamond, light behaves the same way along every axis.
The unit cell of diamond has a lattice parameter of 3.567 angstroms. Eight carbon atoms belong to each unit cell. The density follows from this geometry: 3.52 g/cm3, consistent across all natural and laboratory-grown diamond.
Crystal Habits — What Rough Diamonds Look Like
A crystal habit describes the external shape a mineral tends to grow into. While diamond's internal structure is always the same FCC lattice, the outward form varies depending on the conditions during growth — temperature, pressure, growth rate, and the chemistry of the surrounding environment.
Octahedron. The most common habit for gem-quality natural diamond. An octahedron has eight equilateral triangular faces and resembles two pyramids joined base-to-base. The octahedral faces correspond to the {111} crystallographic planes. Many rough diamonds arrive at cutting houses as complete or partial octahedra. The octahedral shape is significant to cutters because it offers natural symmetry that lends itself to efficient yield — a well-formed octahedron can be sawn in half to produce two round brilliants.
Cube. Cubic diamonds are bounded by {100} faces — six square faces at right angles. Cubic habit is less common in gem-quality rough and more frequent in industrial diamond. Cubic crystals often appear opaque or translucent rather than transparent, though clear cubic rough does occur. The surface of cubic diamonds frequently shows a distinctive stepped or terraced texture caused by alternating growth layers.
Dodecahedron (Rhombic Dodecahedron). Twelve diamond-shaped faces bounded by {110} planes. Dodecahedral diamonds are common and often show curved or rounded faces rather than the flat, sharp faces seen on octahedra. This rounding results from dissolution — the crystal partially resorbed in the mantle before reaching the surface. Many gem-quality rough diamonds are dodecahedral or transitional between octahedral and dodecahedral form.
Combination and Transitional Forms. In practice, many rough diamonds display faces from more than one crystal form — an octahedron with truncated edges showing dodecahedral faces, for example. The relative development of different face types provides clues about the conditions under which the diamond formed and how long it spent in different thermal environments within the mantle.
Flat and Tabular Crystals. Some rough diamonds grow as flattened plates. These thin, often triangular crystals are called macles (discussed below under twinning). They present particular challenges to cutters because their geometry limits the depth available for pavilion facets.
Cleavage Planes
Cleavage is the tendency of a crystal to break along specific planes where atomic bonding is relatively weaker. In diamond, cleavage occurs along the {111} octahedral planes — the same planes that form the faces of an octahedral crystal.
This may seem contradictory: diamond is the hardest natural material, yet it can be split along defined directions. The key is the distinction between hardness (resistance to scratching) and toughness (resistance to fracture). Hardness measures the difficulty of displacing atoms at the surface. Cleavage measures the energy required to separate one plane of atoms from the next. Along the {111} direction, the density of atomic bonds crossing the plane is lower than in other directions, creating a preferential fracture path.
Diamond has four cleavage planes, corresponding to the four unique orientations of the {111} family in the cubic system. A skilled cleaver can position a blade along one of these planes and deliver a precise strike to divide a rough diamond cleanly into two pieces. This technique was the primary method of dividing diamond rough before mechanical sawing became standard in the twentieth century. It remains in use today for specific situations where sawing is impractical — for example, removing a large inclusion positioned near the surface.
Implications for jewellery. Cleavage means diamonds are not indestructible. Thin edges — a knife-edge girdle or the pointed tip of a marquise or pear shape — are vulnerable. A sharp blow along a cleavage direction can chip or fracture the stone. This is one reason gemologists recommend protective settings (such as V-prongs or bezels) for pointed diamond shapes.
Twinning
Twinning occurs when two or more crystal domains grow together in different orientations, sharing a common crystallographic plane. In diamond, the most common twin law is the spinel twin, where two crystal halves are mirrored across a {111} plane. The resulting crystal is called a macle.
Contact twins (macles). A macle is typically a flattened, triangular crystal that looks like two shallow octahedral halves joined at their bases and rotated 180 degrees relative to each other. The twin plane runs through the middle of the stone. Macles are common — they represent a significant portion of natural rough diamond production.
Twinning affects a diamond in several practical ways:
Cutting difficulty. The two crystal domains have different grain orientations. Because diamond's hardness varies slightly with crystallographic direction (a property called differential hardness or directional hardness), a polisher must adjust the polishing direction when moving from one twin domain to another. Twinned rough is harder to polish efficiently than untwinned rough.
Cleavage disruption. The cleavage planes in each twin domain run in different directions. This means a twinned diamond cannot be cleaved through the twin boundary — the cleavage plane in one domain does not continue into the other. Paradoxically, this makes twinned diamonds tougher (more fracture-resistant) than untwinned crystals in some orientations, because a crack cannot propagate straight through the stone.
Internal reflections. Twin planes can act as internal reflectors or cause localised strain, which sometimes appears as a visible grain line under magnification. In clarity grading, twinning wisps — fine, thread-like inclusions associated with twin planes — are a recognised clarity characteristic on GIA reports.
Cyclic and penetration twins. Less common than simple contact twins, these forms occur when multiple twin domains intersect. Star-shaped or five-pointed cyclic twins are known but rare. Penetration twins, where two octahedral crystals interpenetrate, create visually striking specimens that are prized by mineral collectors, though they are rarely used for gem cutting.
Surface Features
Rough diamond crystals carry surface markings that reveal their growth and geological history:
- Trigons — small triangular pits on octahedral faces, oriented opposite to the face outline. They form through natural etching and dissolution. Their depth and sharpness indicate how much resorption the crystal experienced.
- Growth lines — fine parallel lines on crystal faces, tracing successive growth layers. They are most visible on cubic faces.
- Shield-shaped laminae — flat, shield-shaped elevations on dodecahedral faces, recording the transition from octahedral to dodecahedral growth.
- Etch channels — narrow, tube-like channels penetrating the crystal surface, caused by chemical dissolution along defect lines.
These features are removed during cutting and polishing, but they guide the cutter's assessment of the rough stone — revealing internal strain, growth direction, and potential inclusion zones before the first saw cut is made.
Frequently Asked Questions
What crystal system does diamond belong to?
Diamond belongs to the cubic (isometric) crystal system. Its atoms arrange in a face-centred cubic lattice that repeats uniformly in three dimensions, making diamond optically isotropic — its refractive index of 2.417 is the same regardless of the direction light travels through the crystal.
What does a rough diamond look like?
The most common natural shape is the octahedron — two four-sided pyramids joined at their bases. Rough diamonds also occur as cubes, dodecahedra (twelve-faced crystals with often rounded surfaces), and flat triangular macles formed by twinning. Surface features like trigons and growth lines reveal the crystal's geological history.
Why can a diamond be split if it is the hardest material?
Diamond cleaves along four {111} octahedral planes where the density of atomic bonds crossing the plane is lower than in other directions. Hardness measures resistance to scratching; cleavage measures resistance to splitting along specific planes. Diamond cutters have exploited these cleavage planes for centuries to divide rough stones.
What is a diamond macle?
A macle is a twinned diamond crystal — two crystal halves mirrored across a {111} plane, producing a flat, triangular stone. Macles are common in nature and present cutting challenges because the two crystal domains have different grain orientations, requiring the polisher to adjust direction when crossing the twin boundary.
Summary
Diamond crystallises in the cubic system as a face-centred cubic lattice with full three-dimensional symmetry. Its most common natural form is the octahedron, though cubes, dodecahedra, and rounded dissolution shapes also occur depending on growth conditions. Four cleavage planes along the {111} directions give cutters a way to divide rough but also create vulnerability in thin edges of finished stones. Twinning — especially macle formation — complicates cutting by introducing multiple grain orientations, but also increases toughness by blocking crack propagation across twin boundaries. The crystallography of diamond is not separate from its beauty or durability; it is the direct source of both.