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Learn · Lesson 5

Rough: Forms, Resorption, and Reading the Crystal

HOK-DIA-LESSON-00005Monitored

Learning Objectives

Core

A natural crystal can preserve primary growth forms, but it may also undergo partial resorption during long residence in the mantle and especially during transport. The present shape of a rough diamond is therefore not necessarily the shape it had when it grew.

Octahedral, dodecahedral, and other forms should not be treated as simple “quality types.” For planning, it is more useful to understand:

Apply

Begin by separating primary growth form from secondary resorption and breakage. Octahedral, cubic, and transitional geometries can reflect growth, while dodecahedroidal surfaces, trigons, tetragons, frosting, or etch channels often record later dissolution. Rounding produced by chemical resorption should not be confused with mechanical abrasion during alluvial transport.

A macle is a contact twin, not merely “triangular rough.” Cleavage is a crystallographic fracture and must be distinguished from an irregular break or a polished surface. A natural on a polished diamond is a retained portion of the original rough surface; it is not the same thing as an extra facet. These distinctions matter when deciding orientation, division, and finished yield.

A practical rough examination can follow this sequence:

  1. document the external form and major crystallographic faces;
  2. record resorption and other surface features;
  3. locate fractures, inclusions, and strain that could alter the plan;
  4. distinguish a radiation stain or other surface color from color extending through the volume;
  5. only then evaluate possible polished outcomes.

Unusual natural materials further illustrate why a name alone is not enough. Carbonado is a porous, strongly bonded polycrystalline diamond whose primary origin remains unresolved; ballas, bort, and other aggregates have different microstructures and use contexts. “Black diamond” is therefore not synonymous with carbonado, and a natural polycrystalline material is not automatically technically superior to engineered synthetic PCD.

Check Your Understanding

Before accepting a technical or production conclusion, ask:

Conclusion Boundary

Escalate when morphology, an inclusion, or geological context is being used to infer a precise mineral identity, age, pressure-temperature condition, depth, or geographic origin. Escalate production planning as well when internal features, cleavage risk, strain, or color distribution could materially change orientation, division, yield, or the risk of irreversible loss. Those decisions require appropriate analytical and planning methods, with uncertainty stated explicitly.

Common Mistakes

“Dodecahedral rough is simply another primary growth form.”
Many rounded dodecahedroidal forms result from resorption, so the surface features and context must be read before drawing that conclusion.

“Every trigon means the same thing.”
No. Its significance depends on the surface, orientation, and accompanying features.

“A natural on a polished stone is an extra facet.”
No. A natural is retained original rough surface.

“If the rough resembles a known local population, we know the mine.”
No. Similar morphology can develop in different geological systems.

Remember

Rough diamond must be read systematically. The most useful information comes not from one “recognizable” surface feature but from the relationship among form, resorption, internal features, crystallography, and production risk.

Sources

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