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Spinel is a durable, singly refractive mineral with a chemical formula of

MgAl2O4

MgAl2O4

that is prized as a gemstone in a wide range of colors, including red, pink, blue, lavender, violet, green, yellow, brown, and black. Historically, it was often misidentified as ruby or sapphire, but it is a distinct gemstone that is hard enough (7.5-8 on the Mohs scale) to be used in jewelry. Spinel is known for its vibrant and often intense colors, which are caused by trace elements like chromium, iron, and cobalt.

Chemical and physical properties

• Chemical Formula:

  MgAl2O4

  MgAl2O4

  (magnesium aluminum oxide)

• Crystal System: Cubic

• Hardness: 7.5 to 8 on the Mohs scale

• Specific Gravity: 3.58 to 3.61

• Refractive Index: 1.719

• Refraction: Singly refractive

• Cleavage: None

• Fracture: Conchoidal

• Streak: White

Color and appearance

• Colors: A wide spectrum exists naturally, including red, pink, orange, purple, blue, violet, and green, as well as black and colorless varieties.

• Causes of Color: Trace elements are responsible for the color:

  • Chromium: Produces reds and pinks.

  • Iron: Creates purples, oranges, and some blues.

  • Cobalt: Creates rare, vivid blues.

• Appearance: Spinel is often clean with few inclusions in high-quality specimens. The most valuable colors are vivid red, cobalt blue, and bright pink, though pale lavender is a more affordable option.

History and significance

• Historical misidentification: For centuries, red spinel was often mistaken for ruby and is found in many famous historical pieces, such as the "Black Prince's Ruby".

• Modern recognition: It is now recognized as a distinct and desirable gemstone.

• Durability: Its hardness makes it a very durable stone for jewelry.

• Treatments: Natural spinel is typically untreated, as heat treatment is not generally effective at improving color and can damage the crystal.