Cleavage Plane
Cleavage Plane
The crystallographic surfaces along which gemstones preferentially fracture
A cleavage plane is a specific atomic plane within a crystal lattice along which the bonds between atoms are comparatively weak, causing the crystal to break smoothly and flatly when sufficient stress is applied in that direction. Unlike a random fracture, which produces irregular, conchoidal, or splintery surfaces, a cleavage break follows the internal symmetry of the crystal structure precisely, yielding a flat, mirror-like face that reflects light uniformly. Cleavage planes are among the most diagnostically useful features in gemmology and among the most practically consequential for cutters, setters, and conservators.
Crystallographic Basis
Every crystalline mineral is built on a repeating three-dimensional lattice of atoms or ions. The planes of weakest bonding within that lattice define the cleavage directions. These directions are described using Miller indices — the standard crystallographic notation that identifies a plane by its relationship to the unit cell axes. In diamond, the cleavage planes are the {111} octahedral planes, four in number, each parallel to a face of the ideal octahedron. In topaz, a single perfect basal cleavage runs parallel to the {001} plane, perpendicular to the c-axis. Fluorite cleaves in four directions along {111}; feldspar in two directions at near-right angles along {001} and {010}. The number, direction, and quality of cleavage planes are fixed properties of a mineral species and do not vary between specimens.
Cleavage quality is conventionally graded as perfect, good, distinct, or indistinct (or imperfect), reflecting how readily and cleanly the break follows the plane. Topaz and calcite exhibit perfect cleavage; corundum (ruby and sapphire) has no true cleavage, only parting along twin planes — an important distinction discussed below.
Cleavage Planes as Inclusions
Within a fashioned gemstone, a cleavage plane that has already opened — whether partially or fully — appears as a flat, highly reflective internal feature. In the trade it is commonly called a feather, particularly when it is small and partially healed, or a cleavage crack when more pronounced. Under magnification, a true cleavage plane is distinguished from a random fracture by its flatness, its orientation parallel to a known crystallographic direction, and the characteristic step-like pattern that appears when multiple parallel planes have opened in close succession. In diamond, opened cleavage planes oriented parallel to the octahedral faces are a routine finding in rough and in fashioned stones, and their presence, size, and proximity to the surface are central considerations in clarity grading.
Diagnostic Value
Because cleavage directions are species-specific, the orientation and quality of cleavage planes observed under the microscope contribute to gemstone identification. A single perfect basal cleavage, for instance, is immediately suggestive of topaz rather than quartz, which lacks cleavage entirely. In feldspar varieties such as moonstone and labradorite, the two cleavage directions produce characteristic right-angle step fractures that are diagnostic. Gemmologists also use the reflectivity of cleavage faces — which can approach that of a polished surface — to distinguish them from the duller, more irregular surfaces of conchoidal fractures.
Cleavage versus Parting
Parting superficially resembles cleavage but arises from a different mechanism: it occurs along twin planes or planes of structural discontinuity rather than along planes of inherently weak bonding. Corundum exhibits parting along the {0001} basal plane and along rhombohedral planes as a result of twinning, and these surfaces can be mistaken for cleavage by the inexperienced observer. The practical distinction matters because parting planes are not universally present in every specimen of a species, whereas true cleavage planes are an intrinsic property of every crystal of that mineral.
Practical Implications for Cutting and Setting
Knowledge of cleavage plane orientation is indispensable at every stage of a gemstone's working life. Diamond cutters have exploited the {111} cleavage since antiquity to split rough crystals before sawing or bruting, a technique known as cleaving. The cleaver scores a groove along the cleavage direction and delivers a sharp blow with a steel blade; if the orientation is correct, the stone divides cleanly. Misjudging the plane by even a few degrees can shatter the stone rather than cleave it.
For topaz, the perfect basal cleavage presents a persistent hazard: an ultrasonic cleaner, a sharp knock against a hard surface, or even thermal shock during setting can open a cleavage plane invisibly at first, with the crack propagating later under further stress. Setters working with topaz are advised to use bezel or tension settings cautiously and to avoid prong pressure directed along the c-axis. Similar caution applies to kunzite (spodumene), which has two perfect cleavage directions, and to moonstone, whose cleavage planes make it fragile in everyday wear.
During repair and re-polishing, a cleavage plane exposed at or near the surface of a stone can be mistaken for a polished facet, leading to incorrect re-cutting angles. Conversely, a polished facet inadvertently laid parallel to a cleavage plane may exhibit a slightly different lustre or reflectivity from adjacent facets — a subtle but detectable anomaly under raking light.
In Clarity Grading and Treatment
In diamond clarity grading, open cleavage planes that reach the surface are classified as external features (cleavages) and reduce clarity grade more severely than equivalent internal features, owing to their structural vulnerability. Fracture-filling treatments — in which a glass-like resin or lead-based glass is injected under vacuum into open cleavage planes — are applied commercially to both diamonds and coloured stones, most notably rubies, to improve apparent clarity. Gemmological laboratories identify such treatments by the characteristic flash effect (a blue-to-orange iridescence) visible within the filled plane under oblique illumination, and by spectroscopic signatures of the filler material.