In the professional gemstone trade, Spinel is often referred to as “the great impostor” because, for centuries, it was misidentified as Ruby or Sapphire. However, from a chemical and structural standpoint, it is a distinct and highly stable mineral. Spinel is a magnesium aluminum oxide ($MgAl_2O_4$) that crystallizes in the cubic system, giving it unique optical properties that distinguish it from the Corundum family.
1. Atomic Structure and “Single Refraction” Spinel is singly refractive, meaning light entering the stone travels at the same speed in all directions. The Cubic Lattice: Unlike Ruby or Emerald, which are doubly refractive and can show two different colors (pleochroism), Spinel shows one pure, consistent hue regardless of the angle. Octahedral Growth: In its rough form, Spinel naturally grows in perfect double-pyramids (octahedrons). These crystals are often so well-formed at the “Mouth of the Mine” that they are occasionally set into jewelry in their raw, unpolished state.
2. Chemical Variations and Color The “Spinel Family” is defined by isomorphous replacement, where different metal ions substitute for Magnesium or Aluminum without changing the crystal structure. Red and Pink Spinel: Colored by Chromium. These are the most valuable and were historically confused with Rubies (such as the “Black Prince’s Ruby” in the British Crown Jewels). Blue Spinel: Colored by Iron or, more rarely, Cobalt. Cobalt-blue spinels are highly sought after for their intense, electric saturation. Flame Spinel: A specific trade name for vivid orange-red specimens. Pleonaste / Hercynite: Dark green to black varieties rich in iron.
3. Physical and Chemical Properties Spinel is a rugged gemstone, making it ideal for high-wear jewelry. Its hardness is second only to Diamond and Corundum. Property Technical Value Chemical Formula $MgAl_2O_4$ Mohs Hardness 8.0 Specific Gravity 3.54 – 3.63 Refractive Index 1.712 – 1.762 Luster Vitreous (Glassy)
4. Geological Occurrence Spinel typically forms through contact metamorphism, often found in the same marble deposits as Rubies. Because Spinel is chemically resistant to weathering, it frequently accumulates in alluvial deposits (river gravels). For a “Mine-to-Market” operator, East Africa (Tanzania and Madagascar) has become the global epicenter for fine Spinel. The Mahenge region in Tanzania, specifically, is famous for producing “neon” pink and red Spinels that have a clarity and brightness often superior to Burmese specimens.
