Prepare for the DGA by grading in a fixed order, naming every feature with standard trade terminology, and writing a one-sentence justification for each grade. Use worked scenarios to catch common reasoning errors, and use repeat-grading checks to confirm your judgments are consistent, not just plausible.
Grade in a fixed order: why random observation produces inconsistent results
A reliable grading sequence is: confirm identity first, then clarity, then color, then cut and finish. Later judgments often depend on earlier ones, so skipping steps contaminates the whole assessment.
Identity comes first because clarity, color and cut vocabulary only apply once you know you are dealing with diamond and not a simulant. Then work clarity before color: examining inclusions under magnification tells you whether a stone is natural, and girdle condition or fluorescence noticed at this stage can influence the color and finish judgments that follow. Grading color on a stone you have not yet examined internally means you may later discover something that changes your reasoning.
Build the habit of writing each grade down with its justification before moving to the next stage. For example: 'VS2 — the grade-setting feature is a small feather near the girdle; no dark inclusions under the table.' This written chain matters because a defensible grade is one where each step follows from recorded observations, and it gives you a record you can re-check against the same stone later to test your own consistency.
Clarity: separating the grade-setting feature from everything else you see
Clarity grading asks which single feature is most significant face-up, considering size, number, position, nature and relief. The largest inclusion is not automatically the grade-setting one.
Worked scenario: you examine a round brilliant and see a long, thin feather reaching the girdle, plus a tiny dark crystal directly under the table. The tempting mistake is to grade by size alone and let the feather dominate. The better decision is to weigh all five factors: the feather is shallow and near the edge, while the dark crystal sits in a high-visibility position with strong relief against the brilliant background. Depending on overall prominence, the small dark crystal may well be the grade-setting feature. Getting this wrong shifts the grade and misstates the stone's actual face-up appearance.
Train yourself with the correct vocabulary distinctions: an inclusion is internal, a blemish is on the surface; a feather is a break, not a crystal; bearding along the girdle is bruting damage; a natural is an unpolished fragment of the original surface. Plotting matters too — sketch the stone's outline and mark each feature's position and type, because position under the table, on the crown, or at the girdle carries different weight, and a plot forces you to look systematically rather than react to whatever catches your eye first.
Color: using master stones and controlled light instead of eye judgment alone
Color grades are assigned by comparing a stone against master stones under controlled neutral lighting. Memorizing the letter scale without practicing comparisons produces unreliable color calls.
Worked scenario: a student grades a yellowish stone as G under warm shop lighting because it looks 'near colorless' next to a white metal setting. The mistake is grading from appearance in ambient light, which transmits surrounding color to the eye. The better decision is to use a standardized neutral light source and compare the stone pavilion-side against a master series, noting that grading approaches differ — many assess color through the pavilion, while face-up appearance also matters in practice — and to record the lighting conditions with the grade. Under controlled comparison, the same stone may grade clearly lower, and the metal-setting impression had no grading value at all.
Fluorescence complicates this deliberately: a stone with blue fluorescence can appear more colorless in daylight containing ultraviolet, so a yellowish body color may be partly masked under some lighting. This is why the interaction between body color and fluorescence belongs in your notes rather than being an afterthought. When you practice without a full master set, grade stones relative to each other under the same lamp, in the same position, against a neutral background, and record which stone you treated as your reference. Relative consistency is the trainable skill; the letter labels follow from it.
Cut and finish: proportions, polish and symmetry are separate judgments
Cut assessment covers proportions such as table size, crown and pavilion angles, girdle thickness and culet, while finish covers polish and symmetry. Appearance terms like brightness, fire and scintillation describe the result.
Keep the framework of proportions versus finish in two columns. Proportions describe geometry: a steep crown, a large table, a thick girdle or an off-center culet each change how light travels through the stone. Finish describes execution: polish lines, nicks, abrasion, and symmetry faults such as an off-round outline, misaligned facets or an extra facet placed carelessly. A stone can have sound proportions but poor polish, and conflating the two produces a muddled cut description that no reader can act on.
Finish also connects back to clarity: an extra facet is a blemish-class feature that belongs on the plot, so your cut work feeds your clarity reasoning. Practice by describing appearance in the correct terms — brightness for returned light, fire for dispersed color flashes, scintillation for the pattern of flashes as the stone moves — and then attribute each observation to a cause: 'weak contrast pattern consistent with a large table' rather than simply 'dull'. This causal link is what turns an impression into a grade.
Simulant, synthetic, treated: matching observations to the right conclusion
A simulant is a different material imitating diamond; a synthetic diamond is the same material grown in a laboratory; a treatment modifies a natural stone. Each category needs different evidence.
The vocabulary distinction drives everything. Cubic zirconia and moissanite are simulants — different substances. A lab-grown diamond has the same chemistry and crystal structure as a natural one, so loupe observations alone cannot conclusively separate natural from synthetic; definitive separation relies on advanced instrumentation and fluorescence imaging of growth patterns. A treated diamond is natural diamond whose color or clarity was altered. Calling a lab-grown stone a 'simulant', or claiming a loupe can prove origin, are category errors that a precise study plan avoids.
Use this decision table to connect what you observe with what you may conclude:
Treat every row as a pointer, not a verdict. Positive thermal-probe results, for instance, do not distinguish natural from synthetic diamond, and visible doubling in moissanite depends on direction and viewing conditions. The defensible habit is to state the observation, state the category it points toward, and state what further testing would confirm it.
| Observation | Points toward | Reliable conclusion? |
|---|---|---|
| Doubling of opposite facet junctions through the crown | Moissanite (doubly refractive simulant) | Strong indicator for that stone; diamond is singly refractive |
| Quick thermal response on a probe | Diamond or synthetic diamond; excludes many simulants | Does not separate natural from synthetic |
| Colorless, singly refractive, no doubling at any angle | Diamond or cubic zirconia | Probe or further testing needed to separate the two |
| Metallic flash effect inside a surface-reaching break | Glass-filling treatment | Indicator of treatment; document before any clarity grade |
| Growth-related fluorescence patterning | Synthetic growth origin | Requires advanced laboratory equipment to confirm |
Treatments: why you must note the evidence before the grade, not after
Clarity and color treatments alter a stone's appearance, so they must be recorded before any grade is finalized. Laser drilling, fracture filling, and high-pressure high-temperature annealing each leave characteristic evidence.
Laser drilling leaves a visible channel reaching a dark inclusion, sometimes bleached to reduce its visibility; fracture filling introduces a high-refractive glass into surface-reaching breaks, showing a distinctive flash of color that shifts as the stone tilts — different in character from dispersion. High-pressure high-temperature annealing can substantially change body color, particularly in certain types of material, and is not detectable by loupe. A plausible practice mistake is grading clarity first and 'discovering' a filling channel afterward, which invalidates the grade; the better decision is to check for treatment evidence as part of your initial internal examination, before committing to any clarity or color position.
This sequencing matters because disclosure is central to professional practice: a filled clarity feature is not an ordinary inclusion, and treating it as one misrepresents the stone. In your notes, record the treatment evidence, its location, and how it affected your judgment. In paper scenarios and supervised practical settings, this means writing 'surface-reaching break with glass filling — clarity grade not applicable as an untreated stone' rather than silently grading through the filling. The discipline of evidence before grade is the transferable skill here.
A five-week practice sequence with a self-check rubric
Structure practice as weekly cycles: terminology, then clarity plotting, then color comparison, then cut description, then identification and treatments, each ending with re-graded stones and a rubric check.
A realistic adaptable sequence: week one, define and hand-write twenty standard clarity terms and plot three stones; week two, grade four stones for clarity only, writing a one-sentence justification per stone; week three, run relative color comparisons under one fixed lamp, ranking stones before labeling them; week four, describe cut by separating proportions from finish in two written columns; week five, work identification and treatment scenarios from the table above, stating the observation, category and confirming test for each. Adjust the pacing to your own access to stones and equipment.
Use this rubric each week and record the scores; they are learning milestones, not predictions of any exam outcome. Aim for every 'yes' before moving to the next week's focus.
- Consistency: regrade one earlier stone and match your previous grade and justification, or explain any change.
- Vocabulary: every feature named with a standard term — no 'mark', 'spot' or 'flaw'.
- Justification: each grade supported by one sentence naming the grade-setting feature and its position.
- Sequencing: identity and treatment checks recorded before clarity and color grades were finalized.
- Caution: any conclusion about origin or treatment states what further testing would confirm it.
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.