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Spinel (magnesium aluminum oxide)
Color: Red, purple, blue, lavender, green, pink, yellow, orange and black.
Moh’s hardness: 8
Refractive index: 1.712-1.736
Critical angle: 35.5°
Specific gravity: 3.58-3.61
Fracture: Conchoidal, uneven
Dispersion: Moderately high (.026)
Heat sensitivity: None
Birefringence (double refraction): None
Crystal structure: Isometric
Polishing: Alumina on most polishing laps; 50,000 diamond on most common polishing laps, or 50,000 or 100,000 diamond on ceramic.
Comments: Synthetic spinel is available in many colors. Some spinel has photochroic (color change) properties.