Rough Diamond: Reading an Uncut Crystal
Before cutting, a diamond exists as a rough diamond—a crystal, twin, aggregate, or fragment whose external form records both its growth and its later geological history.
Rough is not simply an “unfinished polished diamond.” Its surface may preserve evidence of:
- growth;
- twinning;
- plastic deformation;
- resorption and etching;
- cleavage or irregular fracture;
- volcanic and sedimentary transport;
- natural irradiation.
The most important lesson The present shape of rough is not necessarily its original growth form. It is often the result of what was added to the crystal—and what was later removed from it.
Four processes that shape rough
For practical interpretation, it is useful to distinguish four processes:
- growth—the formation of crystal faces and zones;
- twinning and deformation—a change in crystallographic continuity;
- resorption—the chemical removal of diamond material;
- fracture and later surface alteration—from the mantle to alluvial transport.
Octahedron, cube, and combination habit
An idealized natural diamond is often depicted as an octahedron with eight faces of the {111} family. Natural crystals may also display cubic {100} faces or a combination habit.
The cube/cuboid and cuboctahedron are important because they show that growth does not follow one universal geometric pattern. Temperature, fluid composition, supersaturation, and other variables can change the relative development of faces.
Internal zoning may preserve an earlier habit even when the external surface was later almost completely resorbed.
[VISUAL 13.1: Primary growth habit—octahedron, cuboid, cuboctahedron, and their {111}/{100} faces]
Macle: a contact twin
A macle is a contact-twinned diamond, often flattened and triangular. Its geometry results from the regular crystallographic relationship between two domains and enhanced growth at reentrant angles.
More recent three-dimensional studies of macles further confirm that the flat shape cannot be adequately explained as two finished octahedra simply “glued” together.
For a planner, a macle is important because it:
- may favor shallow or fancy shapes;
- has particular processing directions;
- may have strain and fractures associated with the twin boundary.
Not every triangular plate is a macle: cleavage can produce a similar outline.
Dodecahedroid and tetrahexahedroid: resorption forms
Rounded “dodecahedral” rough in gemological practice is generally not a primary rhombic dodecahedron formed by growth. It is a secondary resorption form, often described as a dodecahedroid.
Experimental and natural studies show that an octahedral crystal can gradually become rounded during dissolution and develop high-index surfaces, including tetrahexahedroidal (THH) forms.
Roundedness therefore does not prove prolonged river transport. Chemical resorption in the mantle or kimberlite system can produce pronounced rounding before the diamond ever reaches the surface.
Trigons, tetragons, and etch features
Trigons are among the best-known triangular surface features of natural diamond. In the final text, they are treated primarily as etch/resorption structures associated with dissolution of {111} surfaces, not as simple evidence of growth.
Their form may depend on:
- temperature;
- pressure;
- the H₂O-to-CO₂ relationship;
- fluid or melt composition;
- crystal defects at which etching begins.
They may be shallow or deep and have flat or pointed bottoms. Their relationship with other etch structures may help establish the relative sequence of events.
Tetragons are analogous four-sided etch features associated with cubic {100} sectors. On a strongly resorbed stone, they can help reconstruct an earlier cubic habit.
A single surface mark is not, by itself, a sufficiently precise geological thermometer or “mine signature.”
[VISUAL 13.2: Etch atlas—trigon on {111}, tetragon on {100}, hillock, etch channel, and later overprint]
Hillocks, frosting, and etch channels
Resorption may create:
- hillocks and teardrop structures;
- finely frosted or matte surfaces;
- smoothly rounded resorption surfaces;
- dissolution channels along defects and fractures.
Their value lies in combination. The order in which features overlap—for example, a newer circular etch pit over an older trigon—can reveal the relative sequence of changes in the fluid or melt.
Cleavage and fracture
Diamond has perfect cleavage planes parallel to the {111} family. A natural or manufacturing-related cleavage surface may therefore be flat and bright.
It must be distinguished from:
- an irregular fracture;
- a polished facet;
- a smooth resorption surface.
A cleavage surface records breakage, but without additional context it does not reveal when the break occurred—in the mantle, during transport, at the mine, or during later handling.
Alluvial features and radiation stains
Sedimentary transport may produce percussion marks, small chips, and additional mechanical rounding. Chemical resorption and mechanical abrasion, however, must not be confused with one another.
Green or brown radiation stains may form when a diamond surface remains in contact with a radioactive mineral or fluid for a long time. They often affect only a very shallow surface layer. A stain on rough is therefore not the same as bodycolor extending through the entire stone.
Naturals
A natural is a portion of the original rough surface intentionally or incidentally left on a polished diamond.
It may be retained to:
- preserve weight;
- maintain the desired outline;
- avoid excessive cutting;
- in a special context, preserve information about surface color.
A natural is not the same as an extra facet. A natural is a preserved portion of the original rough surface; an extra facet is an additional polished facet outside the standard arrangement.
Rough is not a geographical passport
The morphology of a particular mining population may show a statistically characteristic distribution of forms, resorption, or surface features. This does not mean that an individual unknown rough diamond can be assigned reliably to a mine by appearance alone.
The same types of habit, trigons, resorption, and fracture may occur at different deposits.
Rough also does not prove natural origin by itself. A laboratory-grown diamond may have recognizable growth morphologies, but advanced or processed samples require laboratory verification.
Reading rough before planning
A practical examination should follow the same sequence every time:
- shape—octahedron, cuboid, macle, resorbed form, or irregular fragment;
- surface—growth, resorption, and fracture features;
- interior—inclusions, fractures, graining, color, and zoning;
- strain—especially around larger inclusions and twin boundaries;
- risk—surface-reaching fractures, cleavage, and vulnerable points;
- possible products—one or more polished stones.
If the surface is opaque, a planner may create a small window, or locally polished opening, to inspect the interior. This intervention already alters the rough and must therefore be documented.
From geological object to manufacturing plan
Rough shape strongly influences the possible polished shapes but does not determine them completely.
A regular octahedron may support round brilliants well. An elongated fragment may favor an oval, pear, or marquise. A flat macle may point toward a shallower fancy shape. Fancy-color rough may require an entirely different orientation to preserve or intensify color.
The final decision therefore combines:
- crystallography;
- 3D geometry;
- inclusions and strain;
- color;
- expected cut quality;
- polished yield;
- market value and risk.
The detailed manufacturing workflow follows in Chapter 14, and economic optimization in Chapter 15.
Chapter summary
- A rough diamond carries evidence of growth, deformation, resorption, fracture, and transport.
- The octahedron is the archetypal natural growth form, but natural rough displays many other morphologies.
- The cube/cuboid and cuboctahedron reflect development of the {100} and {111} sectors.
- A macle is a contact twin, not merely a triangular piece of diamond.
- Dodecahedroidal and THH forms are generally secondary products of resorption.
- Trigons and tetragons are interpreted primarily as etch/resorption features on their respective crystallographic surfaces.
- Hillocks, frosting, and etch channels provide an additional record of dissolution.
- Cleavage must be distinguished from irregular fracture and a polished facet.
- Alluvial rounding and chemical resorption are not the same process.
- A radiation stain may be extremely shallow and does not necessarily represent the color of the entire stone.
- A natural is a preserved portion of the original rough surface and is not the same as an extra facet.
- Rough morphology alone cannot reliably identify a mine of origin; nor can it, in every case, prove natural origin.
- Professional planning begins with a systematic reading of shape, surface, interior, and risk.
[VISUAL 13.3: Systematic examination of rough—shape → surface → interior → strain → risk → plan]