Gemology Fast Track / Unit 18 of 24 · Major species · 60 min
Diamond, simulants and lab-grown
Goal: Separate diamond from every simulant and know when a lab is mandatory.
10Hardness
2.417RI · SR
0.044Dispersion
3.52SG
Diamond vs simulants
MaterialRISGDisp.Key test
Diamond2.4173.520.044Thermal + electrical tester; sharp edges
Moissanite2.65–2.693.220.104Doubled facets; moissanite tester
Cubic zirconia2.15–2.185.800.060Heavy; fails thermal tester
White sapphire1.76–1.774.000.018Low fire; on refractometer scale
Zircon1.93–1.984.700.039Strong doubling
YAG1.8334.550.028Low fire, SR
GGG1.977.050.045Very heavy
Strontium titanate2.4095.130.190Excess fire, soft, worn edges
Glass1.50–1.702.3–4.5lowBubbles, rounded edges
Quick visual tests
Read-through
Loose round brilliant table-down on printed text: diamond (well cut) — can't read; most simulants — text shows.
Girdle
Diamond: frosted 'bruted' or polished, may show naturals and bearding. CZ/glass: moulded or polished smooth.
Facet edges
Diamond razor-sharp; simulants rounded and abraded with wear.
Lab-grown diamond
HPHTMetallic flux inclusions (magnet may pull), cuboctahedral growth sectors, often blue SW fluorescence with phosphorescence.
CVDLayered growth, strain banding in polariscope, Si-V 737 nm PL. Often post-growth HPHT annealed.
Your ruleSame properties as natural: no handheld classic test separates them. Use a screening device (e.g. type IIa / PL-based) and a lab report for any stone you buy as natural.
Diamond grading (GIA 4Cs, short)
Colour D–Z (D colourless)Clarity FL, IF, VVS1–2, VS1–2, SI1–2, I1–3Cut (rounds): Excellent → PoorCarat 0.01 ct precisionFluorescence noted
You deal in coloured stones, but you'll meet colourless 'diamonds' in trade. Know the simulants; buy diamonds only with a lab report.
Gems in this unit (9)
DiamondFancy colour diamondSynthetic moissaniteCubic zirconiaYAGGGGStrontium titanateLab-grown diamond (HPHT)Lab-grown diamond (CVD)
Full gem records open in the members' Gem database.
Practical
Practise the read-through test on a CZ (cheap). In the Database, filter 'S' (man-made) and read every diamond simulant line.
Unit quiz
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