DGA Diamond Diploma: Identification vs Grading Skills

For the Gem-A DGA Diamond Diploma, train three separate habits: first describe what you observe, second name the feature or property using gemmological vocabulary, third state whether the point being tested is identification (what the material is) or grading (how the diamond compares with standards). Practise with written scenarios where you decide which kind of question is being asked before answering, and check your answers against a rubric that penalises mixed vocabularies.

Simulant, synthetic and natural: three answers that must never merge

A simulant is a different material that imitates diamond; a synthetic (lab-grown) diamond is the same material made artificially. Naming moissanite as 'synthetic diamond', or a lab-grown diamond as a 'fake', are category errors worth drilling out early.

The distinction is definitional, not stylistic. Cubic zirconia, moissanite, synthetic spinel, strontium titanate and glass are simulants: each has its own refractive index, dispersion, hardness and specific gravity, so each can be separated from diamond by standard tests. A lab-grown diamond, whether grown by high-pressure high-temperature (HPHT) or chemical vapour deposition (CVD) methods, is carbon in the diamond structure. Separating it from natural diamond rests on growth features and trace chemistry, not on the basic properties that separate diamond from simulants.

Build a two-stage decision habit. Stage one: is this diamond or a simulant? Use properties such as refractive index (diamond about 2.42, moissanite roughly 2.65-2.69), dispersion, thermal conductivity and hardness. Stage two: if it is diamond, is it natural or lab-grown? Use different evidence, such as fluorescence patterns, inclusions characteristic of natural growth, metallic inclusions in some HPHT stones, or strain and sector zoning seen under magnification. Writing a stage-one conclusion when a stage-two question is asked, or vice versa, is the error to eliminate.

Worked scenario: you examine a bright stone, note strong dispersion and double refraction visible through facets near the girdle, and take a reading consistent with moissanite. A plausible mistake is to write 'synthetic diamond, excellent quality'. The better answer is 'moissanite, a simulant of diamond', followed by the observations that justify it: strong double refraction near the girdle, higher dispersion and a refractive index above diamond's. Why it matters: the two answers describe different materials with different trade treatment, and the correct material name plus the correct category is the defensible conclusion, whereas a substitute such as 'fake' identifies nothing.

• Simulant: a look-alike material of different chemistry (e.g. moissanite SiC, cubic zirconia ZrO2)
• Lab-grown (synthetic) diamond: same carbon diamond structure, artificial origin
• Natural diamond: same structure, geological origin
• Stage-one evidence separates diamond from simulants; stage-two evidence separates natural from lab-grown diamond

Clarity grading: inclusions inside, blemishes on the surface

Clarity grading distinguishes internal features (inclusions) from surface features (blemishes), and the grade depends on size, number, position, relief and nature of the features. Plotting is documentation: every mark on the diagram must correspond to a feature you can describe and locate.

The inclusion/blemish distinction drives both terminology and grading consequences. Inclusions are enclosed or extending-into-the-stone features: crystals of other minerals, feathers (cleavage fractures), clouds of tiny pinpoints, growth tubes. Blemishes are surface features: scratches, nicks on facet junctions, abrasions, extra facets, polish lines. A feather reaching the surface is still described as an internal feature that breaks it, and its depth matters because it raises durability questions in addition to clarity questions. Practise writing one-line feature descriptions with location, such as 'small crystal, table centre, low relief', rather than vague phrases like 'some spots'.

The grading logic is comparative: you judge how the features affect appearance against the standard descriptions of the clarity scale, from flawless and internally flawless down through the VVS, VS, SI and I ranges. Position matters because a feature under the table is more visible than the same feature near the girdle; relief matters because a dark crystal is more apparent than a colourless one of the same size; a cloud in combination can lower a grade more than its parts suggest. Drill the reasoning in that order: nature, size, number, position, relief, then grade.

Worked scenario: a stone shows a small dark crystal near the centre of the table and a nick on one girdle facet junction. The mistake is to merge them into one plotted feature and argue the grade down twice. The better handling is to plot the crystal (inclusion) and the nick (blemish) separately, then assess their combined visibility as the scale requires. Note that the nick alone would rule out an internally flawless-style assessment, since that category permits no surface blemishes, and it is the inclusion's visibility that primarily sets the grade. Why it matters: separate plotting shows which observations carry which consequences, while muddled plotting hides the reasoning even when the final grade is defensible.

Colour grading D-Z: comparison against standards, not memory

Colour grading compares the diamond body colour with master stones under controlled conditions; letter grades describe where the stone falls between neighbouring standards. Fluorescence is recorded separately and must not be folded into the colour judgement.

The D-to-Z scale is ordinal and comparative. D means colourless relative to the master set; each subsequent letter band is defined by comparison, so the skill is placing a stone between two adjacent masters, not recalling what 'G colour' looks like from memory. The conditions that make the comparison valid are the same ones you should be able to name and justify: a neutral background, consistent controlled lighting, the stones table-down, and comparison through the pavilion side. Explaining why each condition exists, for example that body colour is judged along the pavilion path rather than face-up fire, turns a memorised list into understanding.

Fluorescence (the long-wave ultraviolet reaction, typically described from none to strong with colour) is a separate descriptive entry. A blue fluorescence reaction does not upgrade or downgrade the colour grade in itself; conflating the two is a vocabulary error worth keeping distinct in notes and answers. Keep phosphorescence separate too: it is continued glow after the UV source is removed, a diagnostic observation in some identification contexts, not a colour-grade input.

Scenario for practice: a stone appears slightly warmer than your H master but not as warm as I. The plausible mistake is to average mentally and write 'I, because it looks yellowish'. The better decision is to state the placement, between H and I, graded against the masters in the stated conditions, and record the UV reaction as its own entry. Why it matters: the placement answer demonstrates a repeatable method, whereas an impression-based colour answer cannot be checked, reproduced or defended if questioned.

Crystallography with consequences: cleavage, hardness and the octahedral plane

Diamond's perfect octahedral cleavage and anisotropic hardness are not trivia; they explain why diamonds split along specific planes, why resistance to abrasion varies with direction, and why polishing direction matters. These facts support durability and cutting answers, not just theory marks.

Cleavage is a tendency to split along specific atomic planes, in diamond the octahedral planes, producing flat, reflective surfaces. A feather is a cleavage fracture in trade vocabulary, and its orientation relative to facet junctions and the girdle determines whether a blow during setting or wear is likely to extend it. A cleavage plane running to the girdle is a different practical risk from an enclosed feather of the same size in the pavilion. Practise describing orientation, not just presence: 'feather reaching the bezel facet junction, inclined toward the girdle' carries far more information than 'has a feather'.

Hardness anisotropy is the twin concept. Diamond is hardness 10 on the Mohs scale, but resistance to abrasion varies with crystallographic direction, which is why cutters polish certain directions more readily than others and why a polished girdle may behave differently from a bruted one. Connect property to consequence in your answers: the same anisotropy that permits polishing also means wear against diamond can occur in a preferred direction, and it explains directional features such as polish marks. Linking property to consequence converts crystallography from recall into application.

Treatments and lab-grown diamonds: building an evidence chain

For treated and lab-grown diamonds, the trainable skill is matching observations to the specific process that explains them. HPHT treatment, irradiation, annealing, fracture filling, HPHT growth and CVD growth each leave different, sometimes overlapping, evidence; secure answers name the process behind each observation.

Build the chain in one direction: observation, then process, then residual uncertainty. High-pressure high-temperature treatment can modify or remove certain colouration and may alter inclusion appearance; irradiation with annealing can create or modify colour; fracture filling introduces a high-refractive-index glass into surface-reaching cleavages, recognised by flash effects and filler features. HPHT-grown stones may show metallic inclusions and particular luminescence zoning; CVD stones may show layered growth and characteristic strain behaviour. Note where the evidence is conclusive for a trained observer with proper equipment and where it is merely suggestive, and say so: calibrated hedging signals understanding.

A useful study structure is a matrix you build yourself: one row per process, columns for what the process does, what it leaves behind, which standard instruments reveal it, and what remains ambiguous without advanced instrumentation. Filling the matrix from your course materials forces you to find genuine gaps in knowledge rather than rereading familiar pages. Revisit it by reproducing it from memory, then checking only the cells you missed.

A self-check exercise: grade a written stone record

Create written stone records describing inclusions, surface features, UV reactions and placement between masters, then identify and grade each one cold. Score yourself against a rubric that separates correct vocabulary, correct category, and correct comparative reasoning.

Exercise: write five one-paragraph stone records, for example (1) small cloud under the table plus strong blue long-wave fluorescence, no phosphorescence; (2) feather reaching the girdle plus a nick on a lower girdle facet junction; (3) metallic-looking inclusion with unusual luminescence zoning; (4) a stone reading between the F and G masters with no UV reaction; (5) strong double refraction at the girdle with high dispersion. After a day's gap, answer each record with material/category, a feature list using inclusion and blemish terms, and a grade or identification with reasoning. Then compare your answers with your course materials.

Self-check rubric: two points for correct category (simulant vs lab-grown vs natural, or correct grade placement between named masters); two points for correct vocabulary (feather, cloud, blemish, fluorescence used in the right place); one point for ordering the answer as observation, term, consequence. A target of 4-5 out of 5 on each record is a learning milestone showing the three habits are working; a lower score tells you which habit failed, not just that you need 'more revision'. Records where you wrote the right grade with wrong reasoning deserve a rewrite, because the reasoning is what a written exam asks you to display.

• Write five stone records, then answer them a day later
• Score each answer on category, vocabulary and reasoning order
• Rewrite any answer where the grade was right but the reasoning was wrong
• Repeat weekly, adding records on treatments and lab-grown features

An adaptable preparation sequence and readiness checks

Sequence your preparation in four passes: properties and crystallography first, then identification evidence, then grading scales, then combined scenario work. Treat yourself as ready when you can produce an identified, graded answer with reasoning, from a cold written record, without notes.

Pass one covers the physics and crystallography: optical properties, hardness, cleavage, and how the diamond structure produces them. Pass two covers identification: simulants one by one, then natural versus lab-grown evidence, then treatments, using the process matrix above. Pass three covers the grading systems: clarity feature vocabulary and comparative logic, colour placement between masters, and cut-related terminology. Pass four is integration: mixed written records, done to a time limit, marked with the rubric. End each pass with a from-memory reconstruction of its key tables before moving on, and adapt the pacing to how long your course materials give you.

Readiness checks you can actually perform: (1) you can define simulant, synthetic and natural diamond without hesitating; (2) you can plot a described stone's features and defend a clarity grade from nature, size, number, position and relief; (3) you can place a stone between two named colour masters and keep fluorescence separate; (4) you can name the process behind three treatment or growth observations and state what evidence each leaves; (5) you can explain why octahedral cleavage changes a durability assessment. For administrative matters such as current exam formats, dates and entry requirements, consult Gem-A directly rather than relying on secondary sources.

References and further reading

Use these references to explore the concepts and check the latest information from the relevant organizations.

• gem-a.com

Continue your preparation

• Gemmological Association Of Great Britain Dga Diamond Diploma
• Study Guides