Gemological Properties of Black Spinel, Pyroxene and Magnetite in Thai Alluvial Corundum Deposits at Den Chai and Bo Phloi (black spinel: spinel-hercynite; black pyroxene: augite; magnetite: magnetite-ulvöspinel)
Octahedral spinel crystals, irregular and angular prismatic pyroxene crystals with surface cleavage striations and subangular magnetite crystals showing strong magnetism. Photo courtesy of Dr. Seriwat Saminpanya.
Property	Black Spinel	Black Pyroxene	Magnetite
Crystal Habit	Octahedral, broken crystal	Prismatic, irregular, anhedral	Stubby, anhedral
Surface Features	Smooth, granular, pits	Smooth, striation parallel to cleavage direction	Smooth, granular, pits, fracture
Luster of Rough	Dull, subvitreous	Dull, vitreous	Dull, submetallic
Luster of polished surface	Bright vitreous	Vitreous to bright vitreous	Submetallic
Fracture	Chonchoidal, even	Splintery, granular	Granular
Powder Color	Dark grey	Light yellowish grey	Black
Specific Gravity	3.82-3.91 avg 3.86	3.33-3.39 avg 3.36	4.64-5.11 avg 4.79
Refractive Indices	1.785-1.789	α = 1.690-1.700 β = 1.700-1.718 γ = 1.720-1.739	Over the limit of standard refractometer
Optic Character	Isotropic	Biaxial, Positive	Isotropic
Hardness	7.5 - 8	5.5 - 6	7.5 - 8
Ultraviolet Fluorescence	Inert	Inert	Inert
Magnetism	-	-	Magnetic
Polished section (under polarizing microscope)	Transparent to opaque/dark greenish brown to black	Transparent brown to greenish-yellow brown (moderate pleochroism)	Opaque/black
(EAS note: black spinel has higher specific gravity and refractive index than other colors of spinel.)
Table: Saminpanya, Seriwat, Sutherland, F.L. (2008) Black Opaque Gem Minerals Associated with Corundum in the Alluvial Deposits of Thailand. Australian Gemmologist, Vol. 23, No. 6, pp. 242-253.

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