Diamond

Understanding diamond type is important because:

• Treatment response: Type IIa brown diamonds can be HPHT-treated to colourless
• Colour origin: Type determines cause of colour
• Value: Type IIa diamonds are often exceptional
• Synthetic identification: Type Ib common in HPHT synthetics
• Scientific interest: Type reveals formation conditions

Diamond type is determined by:

• Infrared spectroscopy: Nitrogen absorption patterns
• UV-Vis absorption: Colour centres
• Photoluminescence: Characteristic emissions

Standard gemmological tools cannot determine type; laboratory analysis is required.

The GIA colour scale grades diamonds from D (colourless) to Z (light yellow/brown):

• D, E, F: Colourless (most valuable)
• G, H, I, J: Near-colourless
• K, L, M: Faint yellow
• N, O, P, Q, R: Very light yellow
• S, T, U, V, W, X, Y, Z: Light yellow

Beyond Z, stones enter the "fancy colour" category where yellow is desirable.

Accurate colour grading requires:

• Master stones: Comparison set of known grades
• Standard lighting: D65 daylight equivalent
• Neutral background: White or grey grading tray
• Controlled viewing: Table-down, pavilion up
• Multiple observations: Check from different angles

Five factors determine clarity grade:

1. Size: Larger inclusions have more impact
2. Number: Multiple inclusions reduce grade
3. Position: Central inclusions more visible
4. Nature: Type of inclusion (feather vs crystal)
5. Relief: Contrast with surrounding diamond

Many buyers seek "eye-clean" diamonds:

• No inclusions visible to unaided eye
• Typically SI1 and above
• Depends on stone size (larger = more visible)
• Personal perception varies

GIA grades round brilliant cut on five levels:

• Excellent: Maximum brilliance and fire
• Very Good: High performance
• Good: Standard appearance
• Fair: Below standard
• Poor: Significant performance issues

Well-cut diamonds maximise:

• Brilliance: White light return
• Fire: Spectral dispersion
• Scintillation: Sparkle pattern when moved

Poor cuts show:

• Windowing: Light passes through
• Extinction: Dark areas
• Fish-eye: Girdle reflection visible

Standard round brilliant approximate dimensions by weight:

• 0.25 ct: ~4.1mm diameter
• 0.50 ct: ~5.1mm diameter
• 1.00 ct: ~6.5mm diameter
• 2.00 ct: ~8.2mm diameter

Actual dimensions vary based on cut proportions.

Price per carat increases at threshold weights:

• 0.30, 0.40, 0.50, 0.70, 0.90 ct: Minor jumps
• 1.00 ct: Major threshold (20-40% jump)
• 1.50, 2.00, 3.00 ct: Significant premiums
• 5.00+ ct: Exceptional premiums

A 0.99 ct diamond costs significantly less per carat than a 1.01 ct stone.

Fancy colour value (generally, for equivalent saturation):

1. Red: Rarest and most valuable
2. Blue: Very rare; high value
3. Pink: Extremely desirable
4. Green: Rare; valuable
5. Orange: Relatively rare
6. Yellow: Most common fancy; value varies
7. Brown: Most abundant; lowest fancy value

High Pressure High Temperature synthetics show:

• Metallic inclusions: Iron/nickel flux catalyst
• Colour zoning: May show cross-shaped zones
• Fluorescence patterns: Different from natural
• Magnetic: Metal inclusions attracted to magnet
• Type: Often Type Ib (isolated nitrogen)

Chemical Vapour Deposition synthetics show:

• Striated growth: Parallel growth lines
• Brown tint: Often HPHT post-treated
• Unusual fluorescence: Orange or other colours
• Very clean: Few inclusions typically
• Type: Usually Type IIa

Various instruments help identify potential synthetics or simulants:

• Thermal probes: Distinguish diamond from most simulants
• Combined thermal/electrical: Separate diamond from moissanite
• Type screening devices: Identify stones needing further testing
• UV screening devices: Detect unusual fluorescence patterns

Consider laboratory testing for:

• Large, high-value diamonds
• Unusually clean stones
• Unusual fluorescence
• Stones without documentation
• Type II diamonds (more likely treated or synthetic)
• Any stone with atypical features