Gemology Fast Track / Unit 10 of 24 · Major species · 60 min
Corundum: ruby and sapphire
Goal: Identify corundum, its varieties, synthetics, look-alikes and key treatments.
9Hardness
1.762–1.770RI
0.008–0.010Bire · U−
4.00SG
Colour causes
Ruby
Cr³⁺. Red fluorescence. Fe quenches it (Thai/Cambodian darker, weaker glow).
Blue
Fe²⁺–Ti⁴⁺ charge transfer. Fe³⁺ adds green/yellow + 450 nm band.
Yellow
Fe³⁺ or colour centres (Sri Lanka, may fade if irradiated).
Padparadscha
Cr + colour centre / Fe: pink + orange balance.
Inside natural corundum
Rutile silkFine needles in 3 directions at 60°/120°. Causes star when dense.
Hexagonal colour zoningStraight bands following prism faces (natural). Curved = synthetic.
Fingerprints & feathersHealed fractures of fluid droplets.
CrystalsZircon with haloes (Sri Lanka), spinel, calcite (Mogok), apatite.
Boehmite needlesAlong twin planes, intersecting.
Rhomboidal 'roiled' swirlsTreacle-like flow in Burmese rubies.
Treatments you must screen
HeatMost corundum. Signs: partly dissolved silk (dotted), discoid stress fractures with glassy halos, melted crystals. Accepted with disclosure; unheated commands a premium.
Lead-glass fillingLow-grade ruby made 'gem-looking'. Flash effect, flattened bubbles, lower-lustre glass at surface. Fragile: acid/jeweller's torch damage it. Very low value.
Beryllium diffusionOrange/yellow/padparadscha colours. Colour rim visible in immersion; needs LA-ICP-MS/SIMS for Be.
Titanium diffusionSurface blue: colour concentrated at facet junctions and girdle in immersion; repolishing removes it.
Flux healingHeated with borax: glassy residues in healed fractures (common in Mong Hsu ruby).
Synthetics
Flame-fusion (since 1902)
Curved striae, gas bubbles, too clean. Cheapest, most common.
Flux (Chatham, Kashan, Ramaura)
Wispy flux fingerprints, platinum platelets. Can fool beginners.
Hydrothermal (Russian)
Chevron/zig-zag growth, gold platelets. Lab often required.
Value
Ruby: pure red to slightly purplish red, strong fluorescence ('pigeon's blood' is a lab trade term)Sapphire: saturated velvety blue, not too darkUnheated > heated >> glass-filledOrigin premium: Burma, Kashmir, Ceylon
Gems in this unit (20)
RubyBlue sapphirePadparadscha sapphirePink sapphireYellow sapphireOrange sapphireGreen sapphireTeal sapphirePurple sapphireColour-change sapphireWhite sapphireParti-colour sapphireGeuda sapphireStar sapphireStar rubySynthetic corundum (flame fusion)Synthetic corundum (flux / hydrothermal)Synthetic star corundumSynthetic colour-change corundumGlass-filled ruby
Full gem records open in the members' Gem database.
Practical
Open the members' Gem Database and filter 'Corundum' (or use the gem list below). Read every variety's ID line. Then quiz yourself: cover the screen, name three separation tests for ruby vs red spinel vs glass-filled ruby.
Unit quiz
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