Notes on Diamonds & Diamond Grading – Assignments 12–16

Assignment 12: Grading Proportions—Table, Crown, and Girdle

  • Context and focus: Upper-proportion relationships of a standard round brilliant (table, crown, girdle) and how those affect light return, brightness, and overall cut quality. GIA cut grades are influenced by how extreme any single proportion factor is relative to others.
  • Core definitions and basis for comparison
    • Average girdle diameter (AGD): the basis for comparing a diamond’s other proportions, calculated as
      extAverageGirdleDiameter=D<em>extmin+D</em>extmax2ext{Average Girdle Diameter} = \dfrac{D<em>{ ext{min}} + D</em>{ ext{max}}}{2}
      where $D{ ext{min}}$ and $D{ ext{max}}$ are the smallest and largest girdle diameters measured across the stone.
    • Total depth percentage (TD%): depth from table to culet expressed as a percentage of AGD:
      ext{TD ext{%}} = \dfrac{\text{Depth}}{\text{Average Girdle Diameter}} \times 100
    • Table percentage: table diameter as a percentage of AGD:
      Table %=Average Table DiameterAverage Girdle Diameter×100\text{Table \%} = \dfrac{\text{Average Table Diameter}}{\text{Average Girdle Diameter}} \times 100
  • Key components of the upper portion of a round brilliant
    • Table, Crown, Girdle; Star facets and their role in light performance
    • Crown area: crown angle, crown height percentage; Girdle: thickness variations
    • Pyramidal relationship: average girdle diameter anchors all other proportion evaluations
  • Why proportions matter for cut grade
    • GIA cut grade is influenced by how extreme any single proportion factor is; extreme table, crown, pavilion or girdle can lower the grade significantly.
    • Examples of adverse combinations: very large table, very steep crown, very deep pavilion, or very thick girdle can reduce the grade.
  • Procedure and measurements
    • Step 1: Determine Average Girdle Diameter (AGD) from min and max girdle diameters in four directions; use thousandths precision for calculations.
    • Step 2: Determine TD% by measuring depth table-to-culet and applying the formula given above.
    • Step 3: Determine Table Percentage using average table diameter and AGD.
  • Measuring tools and methods
    • Millimeter gauge for loose stones; table gauge for table size (described in Direct Table Measurement).
    • Direct table measurement is highly accurate for loose and mounted stones when equipment is used properly.
  • Methods for estimating table percentage (three main approaches)
    • Flash Method: quick visual estimation under overhead light; category ranges:
    • Small flash ≈ table < 60%
    • Medium flash ≈ 60%–64%
    • Large flash ≈ ≥ 65%
    • Ratio Method: compare two segments—outer section from girdle edge to table edge and inner section from table edge to culet. Common approximate correspondences:
    • 1:1 ≈ 54%
    • 1:1.25 ≈ 60%
    • 1:1.5 ≈ 65%
    • 1:1.75 ≈ 69%
    • 1:2 ≈ 72%
    • For percentages between, interpolate and average results from at least two locations; adjust for non-uniform sides by adding 1% (slight variation) or 2% (noticeable variation).
    • Bowing Method: evaluate eight lines forming the box outline from star facets; assign values based on bowing (53%–67% range) and average.
  • Star facets and star length percentage
    • Star facets extend from table edge toward girdle; star length percentage is the star facet length relative to the girdle-to-table distance, typically 50–55% but can extend to 65% in longer stars.
    • Variation in star facet length affects symmetry and may influence crown-related evaluations.
  • Crown considerations and methods for estimating crown angle
    • Crown angle: angle formed by bezel facets and girdle plane; important for brightness and fire.
    • Crown height percentage: distance from girdle plane to table as % of AGD.
    • Crown angles generally in the 25°–35° range yield attractive, bright diamonds when other proportions are compatible.
    • Two estimation methods:
    • Profile method: evaluate eight bezel facets in profile; use rough angle references (90°, 45°, 30°) to form an average crown angle.
    • Face-up method: compare pavilion mains’ widths at table corner vs bezel point; the greater the difference, the steeper the crown angle.
  • Crown height percentage and its charted relationship
    • Crown height % is derived from table % and crown angle using a Crown Height Percentage Chart (per Assignment 12).
  • Girdle thickness and variations
    • Girdle thickness: width of the girdle; can range from extremely thin to extremely thick; thick girdles add weight without improving appearance and may cause gray reflections; thin girdles risk chipping.
    • Estimated girdle thickness can be described with terms: ETN, VTN, THN, MED, STK, THK, VTK, ETK.
    • For assessment, examine the girdle around the circumference in profile at 10x magnification, focusing on valley positions for thickness in standard girdle; painting/digging-out can alter girdle thickness distribution.
  • Girdle thickness percentage relationship to cut grade
    • Girdle thickness percentage = TD% − Crown Height % − Pavilion Depth % (rounded to the nearest 0.5%).
    • High girdle thickness (>10%) often limits the cut grade to Poor or lower.
  • Lower half length percentage and culet size (to complete the cut grade assessment)
    • Lower half length percentage: length of the lower half facets as a percentage of the distance from girdle to culet; assessed around 8 pairs of lower half facets.
    • Culet size: assessed by looking through the table; categories range from None to Extremely Large; culet size impacts pavilion depth and overall cut grade.
  • Putting it together: final cut grade factors
    • The highest cut grade possible is determined by the parameter in the lowest range; interactions between multiple parameters can further reduce the final grade.
    • Emphasis on face-up appearance rather than raw numerical parameters; two diamonds with the same TD% can look very different due to crown/pavilion/girdle differences.

Assignment 13: Grading Proportions—Pavilion and Culet—and Evaluating Finish

  • The pavilion and culet are the lower half of the diamond; they govern how light travels after entering via the table and crown.
  • Key pavilion measurements and definitions
    • Pavilion depth percentage: distance from girdle plane to culet expressed as a percentage of AGD.
    • Pavilion angle: angle formed where pavilion mains meet the girdle plane.
  • Estimating pavilion depth percentage via reflections (face-up method)
    • Look at the table reflections through table and pavilion mains; the reflection outline (circle or octagon) around the culet helps estimate depth.
    • Use star facet reflections to locate the table reflection; smaller reflections imply shallower pavilions; larger reflections imply deeper pavilions.
    • Typical guide: pavilion depth ~ 43% corresponds to a small table reflection; as reflection fills more of the space, depth increases.
  • Relationship between pavilion depth percentage and pavilion angle
    • Generally, higher TD% correlates with a steeper pavilion angle; extremely shallow (
    • A chart (TD% vs Pavilion Angle) guides expected ranges for well-proportioned stones.
  • Culet size and its effect on pavilion depth and cut grade
    • Larger culet tends to shallow the pavilion slightly; culet size should be minimized to avoid unnecessary weight loss and visual distraction.
  • Girdle thickness percentage and lower half length percentage in pavilion/culet assessment
    • These values feed into the final cut grade; girdle thickness percentage and lower half length percentage must be evaluated alongside pavilion depth and crown height.
  • Finish and its two components: Polish and Symmetry
    • Polish: quality of facet surfaces; rated Ex, VG, G, F, P; affected by finishing process and post-cut blemishes.
    • Symmetry: accuracy of facet alignment and sizing; rated Ex to P; include line symmetry for fancy cuts.
  • Clarity of finish characteristics (polish and symmetry) in mounted stones
    • Mounted stones can complicate finish evaluation; prongs can hide blemishes; use 10x magnification, lighting, and reflections to assess finish.
  • Summary points for finish and pavilion-related decisions
    • Finish contributes to the final cut grade; balance across table, crown, girdle, pavilion, and culet to achieve the best possible grade.
    • Symmetry variations and finish defects can move the grade even when other proportions are strong.

Assignment 14: Grading Fancy Cuts

  • Concept: Fancy cuts involve shapes and cutting styles beyond the standard round brilliant; overall appearance often matters more than strict quantitative parameters.
  • Cutting styles and shapes
    • Three major cutting styles: brilliant, step, and mixed cuts.
    • Brilliant-cut shapes (emerald, princess, radiant, etc.) use brilliant-cut facet arrangements for brightness; step cuts (emerald, lozenge, baguette) emphasize clarity and the trickle of light rather than brightness; mixed cuts combine elements of both styles.
  • Shape terminology and descriptors used by GIA
    • Shape + cutting style: e.g., Oval Brilliant, Cut-Cornered Square, Modified Pear, Round Modified Brilliant, etc.
    • Brand names (Radiant, Princess, etc.) are trade terms; GIA uses standard naming: shape + cutting style, e.g., “Emerald Cut” (step cut) or “Oval Brilliant” (brilliant cut).
  • Fancy-cut components and shape-specific considerations
    • Shape components: belly, wing, head, shoulders, lobes, cleft, keel line, etc., especially important for pears, ovals, hearts, marquises; these regions influence width, durability, and symmetry.
    • Fancy-cut girdles: often scalloped; thicknesses vary around the girdle; gaps in girdle thickness can indicate design priorities (e.g., weight retention vs. durability).
    • Tailoring for durability: French tips (for marquises, pears, hearts) replace large bezel facets at tips with star and upper-half facets.
  • Clarity, color, weight, and finish in fancy cuts
    • Clarity: remains critical; some shapes reveal inclusions more readily, others may conceal certain features, especially at points.
    • Color: evaluated face-down and diagonally for color appearance, with additional face-up observations depending on shape; fancy shapes may show color differently due to geometry.
    • Weight and table: total depth rules differ for fancy cuts; table percentage may exceed round-cut norms; total depth is based on stone width rather than AGD for many fancy shapes.
  • Shape appeal and length-to-width ratio (L/W)
    • Shape appeal is the overall aesthetic in the context of the shape and style; not solely symmetry.
    • Length-to-width ratio (L/W) is calculated as
      ext{L/W} = rac{ ext{Length}}{ ext{Width}}
      and reported as the ratio (e.g., 1.50:1).
    • Typical L/W ranges by shape (illustrative): Emerald 1.50–1.75; Heart 1.00; Marquise 1.75–2.25; Oval 1.33–1.66; Pear 1.50–1.75, etc. “Too long” or “Too short” categories exist for each shape.
  • Table, crown, and girdle in fancy cuts
    • Table size: measured by width across the stone; table percentage is calculated as
      extTable%=Table SizeStone Width×100ext{Table \%} = \dfrac{\text{Table Size}}{\text{Stone Width}} \times 100
    • Crown angles: estimated by lengthwise profile; descriptive terms used: acceptable, slightly shallow, very shallow, slightly steep, very steep.
    • Girdle thickness terms mirror round-cut terminology; girdle thickness may vary across points and should be reported with the range (e.g., MED to STK).
  • Bow-tie and pavilion bulge in fancy cuts
    • Bow-tie effects: a dark area across the center in elongated brilliants; more pronounced with poor alignment or extreme pavilion angles.
    • Pavilion bulge: common in step cuts; excess bulge adds weight but can introduce light leakage and reduce face-up brilliance.
  • Culet in fancy cuts
    • Culet size evaluated similarly to round brilliants, using table view; culet can be None to Extremely Large depending on the cut and shape.
  • Shape appeal considerations
    • Shape appeal measures how well a fancy cut looks compared to others of the same shape and cutting style; related to proportion ranges but also to subjective aesthetics.
  • Practical features for professionals
    • Brilliants and fancy cuts require careful articulation of shape description (e.g., Cushion Modified Brilliant), and awareness that brand terms may be used in trade but are not as universally defined as shape+style terms.
    • Quantum of weight loss in recutting fancy shapes may be higher due to adjustments to yield and shape integrity.

Assignment 15: Estimating Weight, Recutting, and Repolishing

  • Why weight estimation matters in mounted stones
    • Mounted stones require estimation for weight because direct measurement is hindered by mounting; this estimation informs pricing, appraisals, and potential recutting decisions.
  • Weight estimation fundamentals
    • Weight estimation uses standard formulas and adjustment factors for girdle thickness and outline variations to estimate overall weight from dimensions.
    • For round brilliants, weight correction factors range typically 1–12% depending on girdle thickness; for fancy cuts, additional corrections account for girdle outline variations (e.g., high shoulders, bulges, pavilion bulge).
    • Weight correction percentage is applied to the base weight calculation:
      extWeight<em>extcorrected=extWeight</em>extbaseimes(100+k100)=extWeightextbaseimes(1+k/100)ext{Weight}<em>{ ext{corrected}} = ext{Weight}</em>{ ext{base}} imes \left(\dfrac{100 + k}{100}\right) = ext{Weight}_{ ext{base}} imes (1 + k/100)
    • Example: if girdle thickness correction is 3%, correction factor = 1.03.
  • Girdle thickness and weight corrections (rounds and fancy shapes)
    • For rounds: girdle thickness percentage is the thickness expressed as a percentage of AGD (computed as TD% − Crown Height% − Pavilion Depth%). If girdle thickness is very high (>10%), weight estimate increases and cut-grade implications follow.
    • For fancy cuts: need to incorporate girdle outline corrections and possibly L/W-based adjustments; weight correction factors are shape-dependent and reflect how different outlines affect weight yield.
  • Length-to-width ratio (L/W) as a determinant for weight estimation in fancy shapes
    • For elongated fancy cuts (e.g., marquise, pear): L/W ratio drives which adjustment factor to use; charts provide factors for different L/W ranges; if L/W falls between charted values, interpolate between two adjacent factors.
  • Examples of weight estimation (three real-world exemplars)
    • Example 1: Emerald-cut center stone and two emerald-cut side stones; compute L/W ratios, obtain interpolation factors from emerald-cut charts, apply girdle thickness corrections, and sum weights.
    • Example 2 and 3: similar approach with oval and radiant-shaped stones; use correction factors for girdle thickness (thick girdle) and L/W-based factors; sum to obtain total weight (e.g., 2.10 cts, 2.22 cts, 3.73 cts in provided examples).
  • Repolishing and recutting concepts
    • Repolishing: refinishing to fix minor clarity issues or nicks, may yield slight improvements in clarity or finish without drastic weight loss.
    • Recutting: significant; may involve changing cutting style to improve brightness, balance, or color. Weighing the decision depends on potential value gained vs. weight lost.
    • Full recutting involves re-fashioning the diamond to a new style; partial recutting heals specific flaws.
  • Decision framework for recutting
    • Market demand for the new shape and potential weight recovery; the cutter’s assessment of feasibility and weight loss; potential impact on color and clarity; cost of recutting; and whether the final appearance justifies the loss in weight.
    • The trade-off between weight retention and beauty/value: weight is not the only determinant of value; the final face-up appearance matters most.
  • Practical advice for customers and trade
    • Communicate clearly about expected weight loss and appearance after recutting; obtain cutter’s input on the best recut strategy; consider jewelry design implications; and document expectations for color, clarity, and cut.

Assignment 16: Laboratory-Grown Diamonds

  • Overview: Lab-grown diamonds (LGD) share the same physical, chemical, and optical properties as natural diamonds but are produced in controlled labs using HPHT or CVD processes.
  • Technologies used to produce LGDs
    • High-Pressure, High-Temperature (HPHT) method: converts carbon sources into diamonds using high pressures (~5–6 GPa) and high temperatures (≈1300–1600°C) with a metal catalyst; crystal growth is typically cuboctahedral for HPHT diamonds; growth rates vary (days to weeks).
    • Chemical Vapor Deposition (CVD) method: uses hydrocarbon gas (e.g., methane) with hydrogen; low pressures and moderate temperatures (~700–1000°C); growth occurs layer-by-layer with terraces and risers; roughs are tabular and often cuboctahedral-like in growth sectors.
  • Growth forms and crystal morphology
    • HPHT: growth typically cuboctahedral; color outcomes depend on nitrogen content and boron presence; type Ib (yellow) often results from isolated nitrogen; colorless HPHT diamonds are usually type IIa or weak type IIb after nitrogen getters; many HPHT diamonds are treated post-growth to adjust color.
    • CVD: most colorless to near-colorless diamonds are type IIa; post-growth treatments (e.g., HPHT) can modify color; growth patterns show terraces and risers; DiamondView can reveal growth sectors and interruptions; color distribution is affected by growth and post-growth processing.
  • Identification and differentiation from natural diamonds
    • Common diagnostic features: cuboctahedral growth sectors, metallic inclusions from catalysts (in HPHT), cross-polarization strain patterns (natural vs. HPHT/CVD), DiamondView imaging patterns, fluorescence responses under UV, and inscriptions (origin marks) on girdles.
    • HPHT vs. CVD: HPHT tends to show metallic inclusions and specific growth zone patterns; CVD tends to show terrace/risers growth with possible color zoning; both can be cut and mounted identically to natural stones but require laboratory confirmation.
  • Lab-Grown Diamond Reports and disclosure
    • GIA issues a Laboratory-Grown Diamond Report (LGDR) with distinct terminology for growth features (e.g., growth remnant for a crystal, indented crystal surface, etc.). The girdle may be inscribed with “Laboratory-Grown” and the report number for disclosure.
    • FTC guidelines emphasize truthful disclosure; marketing and labeling must clearly differentiate LGDs from natural diamonds.
    • The lab’s screening devices (e.g., GIA DiamondCheck and iD100) aid preliminary screening of diamonds for potential LGD content; laboratories use multiple techniques for conclusive identification.
  • Screening devices and services
    • Screening devices separate natural diamonds from LGDs and simulants; they rely on differences in type classification (natural type Ia vs. LGD type IIa/IIb). Devices include GIA DiamondCheck and iD100; laboratories perform confirmatory testing.
    • Melee analysis service: GIA Melee Analysis Service screens 1,800–2,000 stones per hour for lab-grown content; melee must be round, D-to-Z, 0.005–0.2 ct for analysis.
  • Market evolution and disclosure practices
    • LGDs have become more prevalent; market adoption accelerated from 2000s onward; 2010s saw colorless to near-colorless LGDs entering jewelry.
    • Notable industry milestones: LGDs marketed by Gemesis (now Pure Grown), Sumitomo, Chatham, Apollo Diamond, New Diamond Technology (NDT); 2018–2022 saw expansion and mass production capabilities, including large carat sizes and Melee expansion.
  • Practical identification and measurement notes
    • Cross-polarized light helps detect strain patterns; natural diamonds often show distinct strain bands whereas LGDs (particularly HPHT) may show subdued or no clear strain; CVD may show unique cross-hatched patterns (tatami) in some cases.
    • UV fluorescence: natural diamonds show blue fluorescence in many D-to-Z colored stones; LGDs can show various fluorescence patterns; the presence or absence of fluorescence can aid identification but is not definitive.
  • Important caveats for professionals
    • Disclosure remains critical: LGDs must be clearly identified as such in reports and marketing materials; misrepresentation risks regulatory action.
    • Screening devices are not definitive; laboratories confirm with comprehensive testing; always consider lab verification for high-value stones.

Key Formulas and Quick References

  • Proportion relationships (rounds)
    • extAGD=D<em>extmin+D</em>extmax2ext{AGD} = \dfrac{D<em>{ ext{min}} + D</em>{ ext{max}}}{2}
    • \text{TD ext{%}} = \dfrac{\text{Depth}}{\text{AGD}} \times 100
    • Table %=Avg Table DiameterAGD×100\text{Table \%} = \dfrac{\text{Avg Table Diameter}}{\text{AGD}} \times 100
  • Weight estimation (conceptual)
    • Base weight estimation from dimension-based charts; apply a weight-correction factor from girdle thickness and outline variation:
      W<em>est=W</em>base×(1+k100)W<em>{est} = W</em>{base} \times \left(1 + \dfrac{k}{100}\right)
      where $k$ is the percentage correction (e.g., 3% → factor 1.03).
  • Crown and pavilion considerations (concepts only)
    • Crown angle and crown height percentage contribute to the final appearance and cut grade. Crown height percentage relates to AGD as a percentage of the perceived crown height from girdle to table.
  • Fancy cuts: shape appearance vs. symmetry
    • Length-to-width ratio: extL/W=LWext{L/W} = \dfrac{L}{W}; reported as a ratio like 1.50:1.00.
  • Finish evaluation (polish and symmetry)
    • Polish and symmetry are rated separately on a five-point scale (Ex, VG, G, F, P) and influence the overall cut grade.

Connections to Foundational Principles

  • Proportions govern light performance: Table size, crown geometry, and pavilion depth determine how light is refracted, reflected, and dispersed, thereby influencing brightness, fire, and scintillation.
  • Weight vs. appearance trade-offs: Heavier girdles or extreme proportions can increase carat weight but may detract from face-up size or light performance; optimal cut grades arise from balancing multiple factors.
  • Lab-grown diamonds mirror natural diamonds in fundamental properties but require different identification, disclosure, and grading considerations due to growth characteristics and inclusions.

Practical Implications and Examples

  • Market implications: Consumers increasingly encounter LGDs; testers and jewelers must be prepared to identify LGDs and disclose appropriately to maintain consumer confidence.
  • Real-world decision-making: When a diamond has poor girdle thickness but excellent table and crown, recutting and reframing may improve overall grade and appearance, but weight loss must be weighed against potential price gains.
  • Fancy cuts require nuanced evaluation: For fancy shapes, the overall look (shape appeal) can outweigh strict numeric ranges; shape-specific considerations such as bow-tie, table width, crown and pavilion interactions, and girdle irregularities are especially salient.

Key Terms (glossary excerpts)

  • Average Girdle Diameter, Crown Height Percentage, Crown Angle, Girdle Thickness Percentage, Lower Half Length Percentage, Table Gauge, Table Percentage, Total Depth Percentage, Bowing Method, Flash Method, Ratio Method, Star Length Percentage, Pavilion Depth Percentage, Pavilion Angle, Culet Size, Polish, Symmetry, Symmetry Variations, Shape Appeal, Length-to-Width Ratio, Fancy-Cut Shapes, Brilliants, Step Cuts, Mixed Cuts, Cuboctahedral, Growth Sectors, Terrace, Risers, DiamondView, Type Ia/IIa/IIb, HPHT, CVD, DiamondCheck, iD100, LGDR, Melee Analysis.
Notes
  • For deeper practice, refer to the related charts and tables in the source text (these notes summarize concepts and methods, but the full charts provide exact percentage ranges and interpolation guidance).