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Pelmadulla cobalt blue spinel vs blue sapphire: polariscope and Chelsea filter testing?

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V
Victor Lapidary•905 rep

How can international gem dealers identify rare vibrant cobalt spinels from blue corundum in muddy gem washing baskets using portable optical field gear?

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5 Answers from travellers

✓Accepted Answer

Separating natural cobalt blue spinel from blue sapphire (corundum) in Pelmadulla alluvial pits is straightforward using optical optics. Spinel belongs to the isometric (cubic) crystal system and is singly refractive (isotropic); rotating it 360 degrees between crossed polarizers in a handheld polariscope shows complete darkness (extinction) or anomalous double refraction (ADR). Blue sapphire is hexagonal and doubly refractive, blinking four times light-to-dark per revolution. Under a Chelsea color filter, true cobalt-colored spinel glows vivid neon-red due to cobalt absorption bands, whereas natural iron-colored blue sapphires appear dull grey-green.

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D
Dr Sarath Gem•1890 rep

Check the refractive index with a handheld refractometer: natural spinel reads flat at 1.718, whereas blue corundum reads significantly higher at 1.762-1.770.

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V
Victor Lapidary•905 rep

Under a desk spectroscope, cobalt spinel displays three distinct absorption bands in the orange, yellow, and green regions (545nm, 580nm, and 625nm).

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M
Marta Stones•910 rep

Watch out for synthetic flame-fusion blue spinels mixed into alluvial parcels; synthetics feature curved color striae and lack natural negative crystal inclusions.

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D
Dr Senaka Soil•1820 rep

Natural untreated cobalt spinels with electric saturated blue body color command per-carat prices rivaling unheated royal blue sapphires on local trading benches.

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Ranger Sisira Rock•1790 rep

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