Comprehensive Notes: Diamonds & Diamond Grading Book 2 (Assignments 7–11)

The Path Light Travels

  • Overview: The path of light in a diamond depends on the diamond’s optical properties and its cut design. Light enters, travels inside, and exits toward the eye, producing brightness, dispersion (fire), and scintillation depending on angles, interfaces, and facet arrangement.
  • Reflection (basic principle):
    • When light hits a boundary between two media, part reflects. The law of reflection: the angle of incidence equals the angle of reflection. Angles are measured from the normal.
    • Visual analogy: pool cue ball hitting a cushion; the incoming and outgoing angles relative to the normal are equal.
    • In diamonds, polishing creates smooth, mirror-like surfaces to maximize reflection.
    • Notation: Incident ray angle = Incidence angle; Reflected ray angle = Reflection angle; Normal line N perpendicular to surface.
    • Key terms:
    • Incident Ray, Angle of Incidence i
    • Reflected Ray, Angle of Reflection r
    • Normal N
    • 3D note: Light rays striking a surface from various directions form a 3D cone around the normal; only rays inside the critical angle cone exit without internal reflection.
  • Refraction (color and speed change):
    • When light moves between media with different optical densities, its speed changes and its direction may bend. This bending is refraction.
    • Optical density determines how much light slows: higher optical density → slower light.
    • Refractive index (RI) is a material’s measure of this slowing; in gemology, RI(air) ≡ 1 and RI(diamond) ≈ 2.417.
    • Diamond’s RI means light travels about rac{c}{v{ ext{diamond}}} o RI ext{ factor} ightarrow v{ ext{diamond}} ext{ is } rac{c}{2.417}; explicitly, light in air travels faster than in diamond.
    • Incident-to-refracted bending: light bends toward the normal when moving from a less refractive (lower RI) to a more refractive medium (higher RI). Conversely, it bends away from the normal when moving from higher RI to lower RI.
    • Classic analogy: straw appearing bent in water due to refraction.
    • Key definitions:
    • Refracted angle (angle between refracted ray and normal) = angle of refraction.
    • RI (Refractive Index): measure of how light changes speed and direction when moving between media.
  • Snell’s Law (general relationship):
    • For incident angle i and refracted angle r, with refractive indices n1 and n2, respectively:
      n<em>1sin⁡i=n</em>2sin⁡r.n<em>1 \sin i = n</em>2 \sin r.
  • Light speed in media (comparison):
    • In near-vacuum space: light speed ≈ 3×105 km/s3 \times 10^5\ \text{km/s}.
    • In water: ≈ 2.25×105 km/s2.25\times 10^5\ \text{km/s}.
    • In diamond: ≈ 1.24×105 km/s1.24\times 10^5\ \text{km/s}.
    • After exiting back to air: light speeds up to its original speed.
  • Total Internal Reflection (TIR):
    • When light travels from a higher RI medium to a lower RI medium and hits at an angle greater than the critical angle, refraction ceases and all light is reflected internally.
    • Critical angle (for diamond, relative to air): θc=24.5∘.\theta_c = 24.5^\circ. Materials with higher RI have smaller critical angles.
    • Condition: If incident angle i > θc\theta_c, refraction becomes total internal reflection.
    • Significance for cutters: TIR and the critical angle govern how light travels inside a cut diamond and how much light exits to the crown.
  • Critical angle cone and exit behavior:
    • Light rays striking a surface within the critical angle cone can exit the diamond; rays hitting outside the cone are fully internally reflected.
    • The goal in cut design: ensure light enters crown, reflects within pavilion outside the critical angle, and exits through the crown to maximize face-up brightness.
  • Dispersion (fire) and color splitting:
    • White light entering high-RI materials like diamond separates into spectral colors due to differing refractive indices for violet and red.
    • In diamond, violet light has RI ≈ 2.4512.451 and red light RI ≈ 2.4072.407, giving dispersion ΔRI=RI(violet)−RI(red)≈0.044.\Delta RI = RI(\text{violet}) - RI(\text{red}) \approx 0.044.
    • Dispersion is an optical property that varies by material and is a major contributor to a diamond’s “fire.”
  • Path Length and color appearance:
    • Path length: total distance a light ray travels inside a gemstone.
    • Longer path length → more absorption → deeper color; shorter path → less absorption → lighter color.
    • Path length can be altered by cut design to optimize color appearance, especially in colored diamonds.
  • Light performance attributes and appearance:
    • A diamond’s appearance depends on brightness, fire (dispersion), and scintillation (sparkle and pattern).
    • The cut design maximizes internal and external light reflections to achieve high brightness and color appearance.
  • Brightness (definition):
    • The overall amount of light returned to the observer’s eye, from either internal reflections or facet reflections.
    • Brightness is influenced by luster and polish; higher RI materials typically have higher luster, contributing to brightness.
  • Adamantine luster and surface polish:
    • Adamantine: highest luster among transparent gemstones; diamonds exhibit adamantine luster due to high RI and excellent polish.
    • Percentage of light reflected at a well-polished surface at normal incidence: about 17% reflected, 83% transmitted for a well-cut diamond vs. glass (4% reflection).
  • Scintillation and fire patterns:
    • Scintillation comprises sparkle and pattern of light and dark areas on the face-up view.
    • Pattern refers to the arrangement and contrast of bright and dark areas; the cutter’s decisions influence the face-up pattern.
  • Summary of Path Light Travels:
    • Light enters a diamond, interacts with internal/external facets based on cut design, and exits to the eye.
    • The interplay of reflection, refraction, dispersion, path length, and critical angle governs the diamond’s brightness, fire, and scintillation.

The Path Light Travels (Continued) — Development of GIA Cut Grading System

  • GIA’s Cut Grading System foundations:
    • Proportions: angles and relative measurements of polished gem; relationships between angles and facets.
    • Predictive power: required to predict brightness and fire as perceived by most observers under standard lighting.
    • Validation: extensive observational testing (>$70,000 observations) across various proportions and lighting conditions to validate the model.
    • Result: a predictive cut grading system with a range of proportion combinations allowed within a grade; not a single ideal set.
  • GIA Round Brilliant Cut Grades:
    • Five grades: Excellent (Ex), Very Good (VG), Good (G), Fair (F), and Poor (P).
    • Design considerations include: face-up appearance, design quality, and craftsmanship (polish and symmetry).
    • Emphasis: a well-cut diamond should be bright, fiery, sparkling, and have pleasing face-up appearance.
  • The Grading Environment:
    • Consistency in lighting and environment is critical for repeatable grading results.
    • DiamondDock provides daylight-equivalent diffused fluorescent lighting for brightness and face-up pattern, and LED lighting for fire evaluation.
    • Diffused fluorescent lighting emphasizes brightness and pattern; spot lighting emphasizes fire.
  • The Grading Process (overview):
    • Step 1: Clean and place the diamond face-up in grading tray; evaluate brightness under fluorescent light; assign brightness grade (Excellent to Poor).
    • Step 2: Switch to spot lighting to evaluate fire; assign fire grade (Excellent to Poor).
    • Step 3: Initial cut grade is based on the lowest rating among brightness, fire, and pattern.
    • Step 4: Detailed evaluation of proportions; Step 5: assess symmetry and polish to arrive at the overall cut grade.
    • Key concept: cut grade reflects face-up performance, design quality, and craftsmanship.
  • The Diamond Cutting Process (overview of 4P and 5P):
    • 4P: Sawing, Bruting, Coning, Faceting (and Table/ Crown/ Pavilion facets work).
    • 5P: Adds Blocking (initial placement of the first 17–18 facets).
    • Laser automation: modern factories use 4P and 5P machines; can expand to additional processes like Bruting Fancy Shapes and Laser Bruting.

Diamond Cutting

  • The History of Diamond Cutting
  • Early History:
    • Diamonds first mined in India (800–600 BCE); used in jewelry by Romans by ~100 AD.
    • By 1200 AD, diamonds in European royal jewelry.
    • 14th century: cutting began in India and Europe; early cutting was superficial polishing.
    • Peak of India’s diamond trade in the 14th century.
  • Cleaving and the shift to faceting:
    • By mid-1400s, diamonds could be split along cleavage planes; cleaving limited cutter options.
    • 15th–16th centuries: improvements in tooling and motion (smooth rotary motion with cast-iron polishing wheels) enabled more facets.
  • The New Era (late 19th–early 20th century):
    • Inventions in bruting (1870s) by Morse and Field; motorized diamond saw (1900) by Loesser.
    • Modern mass production possible via bruting and saws; mass production of faceted diamonds.
    • The Indian diamond cutting revival occurred around the turn of the 20th century.
  • The Evolution of Diamond Cuts (through history):
    • Early cuts followed octahedral rough; point cut and table cut dominated early periods.
    • Rose cut, single cut, double cut, old mine cut, old European cut – all transitional to modern brilliant cuts.
    • The rise of the modern round brilliant cut in the late 19th to early 20th century.
    • Fancy shapes (trillion, princess, radiant, etc.) developed to maximize weight retention and color.
  • The Cutting Process and Technology (modern):
    • Planning: mapping rough, considering crystal directions, surface features, inclusions; use of planning software and 3D mapping; emphasis on weight retention and market value.
    • Dividing the Rough: kerf marks, cleaving, and laser-assisted segmentation; rough mapping guides cutting directions.
    • Sawing: rotary saws historically; laser sawing introduced in late 20th century with Nd:YAG lasers (532 nm) and water-jet-guided variants for better control and weight retention.
    • Bruting: shaping rough to rounded girdle; automated bruting since the late 1980s; laser bruting for fancy shapes.
    • Coning: forming pavilion angle to minimize contact with the wheel; automation and coning on rings and scaifes.
    • Polishing: final surface finishing with a diamond-coated scaife; tang enables precise angling during polishing.
    • Blocking: establishing the crown/pavilion symmetry; often the first 17–18 facets.
    • Brillianteering: final facet polishing (stars and upper/lower halves); often performed by a skilled operator after automation.
    • Quality Control: post-cut evaluation for brightness, fire, pattern, symmetry, polish; used for ensuring consistency with buyer requirements.
  • The 4P and 5P laser automation (modern advancements):
    • 4P: Sawing, Bruting, Coning, Faceting.
    • 5P: Adds Blocking; can perform additional steps; laser-based systems can perform multiple steps with minimal human intervention.
  • Continuous technological improvement:
    • Israel and India are at the forefront of planning and laser technologies.
    • Surface and inclusion mapping has advanced; strain mapping via polariscope improved.
  • Key terms to know:
    • Bruting: shaping rough to outline; traditional bruting uses two diamonds rubbed against each other to shape girdle.
    • Dop: diamond holder used during processing.
    • Kerf: notch cut into rough to prepare for cleaving.
    • LMR: laser manufacturing remnants; can be a blemish or may be visible.
    • Bearding: tiny feathers at girdle; a girdle fringe effect.
    • EF: Extra facet; not required by cutting style; can be removed or kept; weight retention is a factor.
  • Fancy shapes and trends:
    • Triangular (trillion), princess, radiant, old mine, old European; modern and fancy shapes offer different light performances and weight retention capabilities.
    • The round brilliant dominates the market; fancy cuts provide weight retention advantages and color appearance control.
  • Concluding notes:
    • The cutting process blends historical techniques with modern laser and mapping technologies to achieve optimized light performance and color appearance while balancing weight retention and durability.

Finding and Identifying Clarity Characteristics

  • Core idea: Clarity characteristics are internal inclusions or surface-reaching blemishes that affect a diamond’s appearance and value. They also help identify a stone.
  • Two main types of clarity characteristics:
    • Inclusions: features enclosed within the diamond or extending into it from the surface.
    • Blemishes: external features confined to the surface.
  • The purposes of clarity characteristics:
    • Determine the clarity grade (size, number, location, relief, nature).
    • Help determine a stone’s value.
    • Help identify a stone (unique pattern of characteristics).
  • Looking for clarity characteristics:
    • Begin with face-up view at 10x magnification using a corrected triplet loupe or microscope.
    • Use a systematic approach to locate and identify all characteristics.
    • Practice across many diamonds to develop skill.
  • Tools and techniques:
    • Triplet loupe (fully corrected for color and distortion) for 10x observation.
    • A gemological microscope with darkfield (side illumination against a dark background) and brightfield illumination (illumination from below) can be used.
    • Final grading is based on appearance at 10x magnification.
  • Inclusions: internal inclusions and their subtypes (examples from transcript):
    • Crystal (Xtl)
    • Needle (Ndl)
    • Pinpoint (Pp)
    • Cloud (Cld)
    • Twinning Wisp (TW)
    • Internal Graining (IG)
    • Grain Center (GC)
    • Surface-reaching inclusions: Feather (Ftr), Etch Channel (EC), Bearded Girdle (BG), Bruise (Br), Knot (K), Patch (Patch), Chip (Ch), Cavity (Cav), Indented Natural (IN)
    • Inclusions caused by treatment: Laser Drill-Hole (LDH), Internal Laser Drilling (ILD)
  • Blemishes: surface-only issues like Nicks, Abrasions, Scratches, extra facets, polish marks, lizard skin, polish lines, burn, etc.
  • Inclusions caused by crystal structure: Natural (N) and related growth features; naturals as signs of efficient cutting; growth marks such as trigons; etc.
  • Surface graining (SG): transparent lines on the surface indicating crystal irregularities.
  • Other features: inscriptions (laser-engraved), mounting features, laser manufacturing remnants (LMR).
  • Internal inclusions resulting from crystal structure:
    • Twinning Wisp (TW), Internal Graining (IG), Grain Center (GC).
  • Inclusions caused by treatment:
    • Laser drill-holes (LDH)
    • Internal laser drilling (ILD)
  • Plotting clarity characteristics:
    • A plot maps clarity characteristics by crown (left) and pavilion (right) diagrams using color inks (green for blemishes, red for most inclusions, black for extra facets and mounting notation).
    • Crown and pavilion plots, plus surface-reaching inclusions, are marked accordingly.
  • Examination protocol (step-by-step quick guide):
    • Clean the diamond and set up in a neutral environment.
    • Use 10x magnification to identify clarity characteristics.
    • Distinguish surface vs internal by focusing at various depths and by observing reflections.
    • Use wedge technique to divide the stone into eight sections and systematically scan each wedge (pavilion first, then crown).
    • Use the 10x loupe to determine overall visibility and assign a clarity grade.
  • Key terms and definitions to remember:
    • Inclusion: clarity characteristic totally enclosed in a polished gemstone or extending into it from the surface.
    • Blemish: clarity characteristic confined to the surface.
    • Inclusions can be visible face-up at 10x magnification; blemishes are surface-level features.
    • The plot (crown/pavilion diagrams) is read to interpret and identify clarity characteristics.
  • Practical tips:
    • Always keep the diamond clean.
    • Hold with tweezers to avoid skin oils; use a pointer to distinguish dust from tiny inclusions.
    • In darker or lighter areas, use different illumination to reveal surface-reaching inclusions.
    • Practice with both a microscope and a loupe; always base grade on 10x appearance.

Grading Clarity

  • Five clarity factors determine the impact of a characteristic on the overall grade:
    • Size
    • Number
    • Location (Position)
    • Relief (contrast between inclusion and host)
    • Nature (type and effect of the inclusion)
  • The GIA clarity grading scale includes 11 grades: Flawless (FL); Internally Flawless (IF); Very Very Slightly Included (VVS1, VVS2); Very Slightly Included (VS1, VS2); Slightly Included (SI1, SI2); Included (I1, I2, I3).
  • Callable vs Non-callable clarity characteristics:
    • Callable: affects clarity; visible at 10x.
    • Non-callable: does not affect clarity; can exist on a Flawless diamond (visible above 10x).
  • Step-by-step clarity grading procedure (overview):
    • Systematic wedge examination (eight wedges) using 10x magnification.
    • Identify and plot clarity characteristics.
    • Initial impression strongly influences final grade; then a detailed review.
    • Consider 5 clarity factors in conjunction when determining the grade.
    • Use plots to document grade-setting inclusions and other significant features.
  • The five-factor framework details:
    • Size: larger inclusions usually lower the grade; two larger inclusions may determine the grade even if smaller inclusions exist.
    • Number: more inclusions or reflections raise visibility and potentially lower the grade.
    • Location (Position): inclusions under the table are more visible; those under crowns or near girdle are harder to see.
    • Relief: the contrast of the inclusion relative to the host (color, brightness) influences visibility; colored inclusions are often more visible.
    • Nature: type of inclusion; e.g., surface-reaching vs internal; internal graining, twinning wisps, and growth features influence grade differently.
  • Plotting and grade-setting:
    • A plot is used to map clarity characteristics; grade-setting inclusions are plotted first (often laser drill-holes), then other inclusions, naturals, and extra facets.
    • The reading order of a plot lists characteristics by importance; naturals and extra facets often plotted last.
  • Tips on grade interpretation:
    • An eye-visible inclusion can push a grade down (e.g., I1 with eye-visible features).
    • Even if a diamond has eye-visible inclusions, VS or SI grades may still be possible depending on the combination of five factors.
    • The depth of an inclusion, distance from the surface, and width will influence the grade.
  • The GIA clarity scale hierarchy (relative rarity):
    • FL < IF < VVS < VS < SI < I (with I1 < I2 < I3 in many catalogs; I3 sometimes used informally but not always part of standard GIA grading) – generally: FL and IF are rare; I ranges are more common in industrial or lower-grade diamonds.
  • Reading a plot and matching to the grade:
    • The plot’s clarity symbols correspond to the types of inclusions and blemishes (e.g., LDH, Ftr, Br, etc.).
    • The plotting key helps interpret the locations and types on crown/pavilion diagrams.
  • The importance of experience and practice:
    • Practice grading across various stones and use the wedge method to ensure a comprehensive search for clarity characteristics.
    • Practical experience improves consistency and accuracy in grading clarity.

Grading Color

  • Core concept: Color grading assesses the diamond’s hue, tone, and saturation, which determine the diamond’s color appearance and value.
  • The D-to-Z range (normal color range):
    • Color are graded in a scale from D (colorless) to Z (light yellow, brown, or gray).
    • D-to-Z grading is generally used for stones in the normal color range; beyond Z, colored diamond grading applies.
  • The 27 hues in GIA’s color system:
    • The color system uses 27 hues including red, orange, yellow, green, blue, violet, and their mixes (e.g., pink, orange-red, blue-green, etc.).
    • Hue is the primary color family; tone and saturation refine the color (depth and intensity).
  • Tone and Saturation:
    • Tone: degree of darkness or lightness of color (range from very light to very dark).
    • Saturation: strength or intensity of color (how vivid the color is).
  • The color globe (three-dimensional):
    • Hue on the horizontal plane; Tone on the vertical axis; Saturation on the horizontal axis.
  • The masterstones and masterstone sets:
    • Masterstones are color-comparison diamonds that define color grade boundaries for the D-to-Z range.
    • They are used face-down in standard color grading; for colored diamonds, grading is done face-up.
    • Split-grade masterstones (e.g., H-I) can represent midpoints between grades.
  • The master-eye effect (color grading nuance):
    • When a diamond sits near a masterstone on both sides, the apparent color may shift due to the viewing side (master-eye effect).
    • Graders must account for this optical illusion and use consistent procedures to avoid misgrading.
  • Grading colored diamonds (special system):
    • Colored diamond grading uses reference stones and leads to a different set of terms, not simply D-to-Z.
    • Graders compare stones side-by-side under controlled lighting; a wide variety of masterstones and colored diamond references are used.
  • Estimation and masterstones:
    • Estimation: color grade can be estimated by comparing face-down (best for color) and face-up positions; not as precise as masterstones.
    • For precise color grading, masterstones are preferred; synthetic CZs are cheaper but do not perfectly mimic color of diamond.
  • The color grading environment and equipment:
    • White or neutral surface; DiamondDock or other controlled lighting; daylight-equivalent fluorescent lighting is standard for color grading.
    • The environment should minimize hue shifts and reflections that could influence color judgment.
  • Fluorescence and its role in color:
    • About one-third of natural diamonds show some fluorescence under UV light.
    • Fluorescence is described on the report (None, Faint, Medium, Strong, Very Strong) and, if Medium or stronger, its color is noted.
    • Color grading is performed prior to measuring fluorescence; color can shift under UV exposure.
  • Colored diamonds:
    • Colored diamonds require separate grading protocols; hue, tone, saturation are used to describe color with precise terms (Fancy Light, Fancy Intense, Fancy Vivid, etc.).
    • The presence of color is the primary determinant of value in colored diamonds.
    • The grading reports may include a graphic description of hue, tone, and saturation variations; the report does not include a cut grade for colored stones.
  • In practice:
    • The color grade is influenced by hue, tone, saturation, distribution of color, and the influence of cut on color appearance.
    • It is essential to separate color from brightness and ensure a standard environment for color grading consistent across graders.
  • Describing diamond colors:
    • GIA uses precise terms to describe color, avoiding trade terms that can be ambiguous.
    • The description includes hue (predominant color), tone, saturation, and distribution (even vs uneven).
  • The value impact:
    • The bigger the hue’s intensity (saturation) and the rarer the hue, the higher the price.
    • The big price differences occur at key transitions (D to E at the top end; boundary changes in brown/gray range are nuanced).
  • Final notes:
    • The color grading system provides a universal, repeatable framework for describing color across the market.
    • Colored diamonds are graded in a separate system that recognizes their distinct value drivers (hue, tone, saturation) and often uses face-up grading.

For Further Reading (Optional Reference)

  • The transcript includes recommended articles from Gems & Gemology and GIA Research & News for deeper understanding of cut-grade research, modeling, and historical development.

Summary of Key Concepts (Condensed)

  • Light behavior in diamonds is governed by reflection, refraction, total internal reflection, and dispersion.
  • Cut design controls how light travels inside a diamond, influencing brightness, fire, and scintillation.
  • GIA’s Cut Grading System is based on extensive research and observation, incorporating a range of proportion combinations for each grade.
  • Clarity grading is determined by five factors (size, number, location, relief, nature) and uses a wedge-based systematic assessment to locate and plot clarity characteristics.
  • The GIA color system uses a D-to-Z scale for the normal color range and a separate system for colored diamonds, with masterstones guiding color grading and a master-eye effect to consider viewing bias.
  • Fluorescence, distribution of color, and color appearance can influence the final grading outcome, especially in colored diamonds.
  • Modern diamond cutting blends ancient techniques with advanced technology (planning software, mapping, laser sawing, laser bruting, automated coning and brillianteering) to optimize yield and appearance.
  • Practical grading considerations emphasize standardized lighting, neutral backgrounds, and controlled environments to ensure repeatable, reliable grades.