Introduction
Every polished diamond begins as a rough crystal — an uncut stone whose external shape, internal clarity, and crystal structure dictate what the cutter can make of it. The rough is the raw material, and reading it correctly is the first and most consequential decision in diamond manufacturing.
A well-formed octahedral crystal with few inclusions might yield a large, high-quality round brilliant. A flattened macle may produce two thinner stones, or a single marquise. An irregularly shaped fragment might be suitable only for small melee goods or industrial use. The shape of the rough is not cosmetic — it is the blueprint.
Understanding rough diamonds matters for consumers, too. It explains why certain shapes cost more per carat than others, why two stones of identical polished weight can differ dramatically in price, and why the diamond industry begins not at the jewellery counter but at the sorting table.
Crystal Habits: How Rough Diamonds Grow
Diamond crystallises in the cubic crystal system, but the external shapes — called crystal habits — vary depending on formation conditions, particularly temperature and the chemistry of the growth environment. Four primary habits account for the vast majority of gem-quality rough.
Octahedra are the most prized crystal form: eight triangular faces forming a shape resembling two pyramids joined at their bases. Well-formed octahedra are the ideal starting point for round brilliant cutting because their geometry allows the cutter to orient the stone for maximum light performance. Many of the world's most famous diamonds began as large, clean octahedra.
Macles — also called twinned crystals — are flattened, triangular crystals formed when two octahedral crystals intergrow along a shared plane. Macles are common and present a specific challenge to cutters: the twinning plane creates internal grain directions that complicate polishing. However, their flat profile makes them well-suited to certain fancy shapes, particularly hearts, pear shapes, and marquises.
Cubes are less common than octahedra and tend to form at lower temperatures. Cubic rough is often translucent rather than transparent, with a fibrous or cloudy internal structure that makes many cubes unsuitable for gem use. Clear cubic rough does occur, but it is the exception.
Dodecahedra — twelve-sided crystals with rounded, lens-shaped faces — are not a primary growth form. They result from dissolution: an octahedral crystal that has been partially resorbed by mantle fluids during residence in the mantle or during transport. Dodecahedral rough is common and, despite its rounded appearance, can yield excellent polished stones.
Most rough diamonds do not present as textbook geometric forms. Broken fragments, distorted crystals, and aggregates are common. The sorter's task is to assess what each piece of rough can realistically become.
Reading the Rough: What Cutters See
Before any cutting begins, a rough diamond is studied — sometimes for days or weeks in the case of large, valuable stones. Modern planning uses 3D scanning technology to map the crystal's external geometry and internal features with sub-millimetre precision.
The cutter evaluates several factors:
Shape and symmetry. A well-formed octahedron with good symmetry is the simplest starting point. Irregular shapes require more creative cutting plans and often sacrifice yield for quality.
Inclusions. Their position, size, and type determine whether they can be cut away or must remain in the polished stone. An inclusion near the surface may be removed by shallow polishing. One at the geometric centre may be unavoidable regardless of how the stone is oriented.
Grain direction. Diamond is hardest along certain crystallographic directions and softest along others. The cutter must orient the stone so that each facet can be polished efficiently. Macles, with their twinned grain, require careful handling because the polishing direction changes across the twin plane.
Colour. If the rough shows body colour — yellow tinting from nitrogen, for instance — the cutter considers how shape and proportions will affect the face-up colour of the polished stone. Deeper pavilions can concentrate colour; shallower proportions can dilute it.
Fluorescence. Strong fluorescence in the rough may influence cutting decisions if the market for the intended grade discounts fluorescent stones.
Yield: Why Rough Shape Determines Polished Size
The concept of yield — the percentage of rough weight retained in the polished stone — is central to diamond economics and is governed primarily by the shape of the rough.
A round brilliant cut from an octahedral rough typically retains 40 to 45 percent of the original weight. More than half the crystal is ground away during cutting. This low yield is the primary reason round brilliants command a price premium per carat: buyers are paying not just for the stone they see, but for the rough that was sacrificed to create it.
Fancy shapes often yield significantly more. A princess cut — a square modified brilliant — can retain 60 to 80 percent of the rough, depending on the crystal. Emerald cuts, with their step-cut faceting, are efficient for elongated rough. Pear and oval shapes suit irregular or elongated crystals that would lose excessive weight if forced into a round.
This relationship between rough geometry and polished yield means that the choice of polished shape is not purely aesthetic. It is an economic calculation that begins with the crystal in hand. A flattened macle that would yield a 0.80 ct round brilliant might produce a 1.20 ct pear shape — crossing the psychologically important one-carat threshold and substantially increasing the stone's market value.
Sorting: From Mine to Market
Rough diamonds emerging from a mine are a heterogeneous mixture: gem-quality crystals, near-gem material suitable for lower-quality jewellery, and industrial-grade stones fit only for abrasive and cutting applications. Sorting this mixture is one of the diamond industry's most specialised disciplines.
The modern sorting system, refined by De Beers over more than a century, classifies rough into over 11,000 categories based on size, shape, quality, and colour. Each category carries a different per-carat price, and skilled sorters — trained over years — assign each stone to its appropriate category by hand, assisted by mechanical sizing screens and optical scanning equipment.
Size is the first sort criterion. Rough is passed through progressively smaller sieves, separating stones into weight classes. Large stones — anything above roughly 10.8 carats — are evaluated individually.
Shape separates octahedra from macles, cubes, fragments, and cleavage pieces. Each shape class has different cutting potential and therefore different value.
Quality assesses clarity: the number, size, and position of inclusions visible under magnification, and whether the rough is transparent, translucent, or opaque.
Colour ranges from colourless through progressively saturated yellow to the rare fancy colours — blue, pink, green — that are sorted and valued separately.
The sorting process determines the rough's initial price and its destination. Top-quality colourless octahedra go to the world's premier cutting centres. Lower-quality material may be cut in higher-volume facilities where labour costs are lower and the margins are thinner.
From Rough to Polished: A Preview
The transition from rough to polished diamond involves five principal stages — planning, cleaving or sawing, bruting (shaping the girdle), faceting, and final polishing — each of which is covered in detail elsewhere in this encyclopedia. But the decisions that matter most are made before any of those stages begin, when the cutter studies the rough and decides what to make of it.
A single large rough crystal might be cut as one stone, maximising carat weight. Or it might be split into two or more stones, sacrificing total weight to achieve higher clarity grades in each piece. A 10-carat rough with a central inclusion might yield a 4-carat VS1 stone and a 3-carat VS2 — or a single 6-carat SI1, depending on the cutter's judgement about where value is maximised.
These trade-offs are invisible in the finished jewellery piece, but they are the reason two diamonds of identical polished specifications can have very different market histories — and why understanding rough is part of understanding diamonds.
Summary
Rough diamonds are the starting point of every polished stone, and their crystal habits — octahedra, macles, cubes, dodecahedra — govern what shapes can be cut, how much weight is retained, and what quality grades are achievable. The sorting and evaluation of rough is a century-old discipline that classifies stones into thousands of categories before a single facet is placed. Understanding the rough is understanding the constraints and possibilities that define every diamond in the market.