Spinel is a crystalline mineral, often found in a variety of colors, and prized as a gemstone. Chemically, it's a magnesium aluminum oxide (MgAl2O4) and is known for its hardness and durability, ranking 8 on the Mohs scale. Spinel is frequently mistaken for rubies and sapphires, especially the red and blue varieties, and can be found in igneous and metamorphic rocks.

Here's a more detailed description:

Appearance:

• Color: Spinel exhibits a wide range of colors, including red, pink, lavender, violet, blue, green, brown, and black.

• Crystal Structure: It typically forms in the isometric (cubic) system, often as octahedral crystals.

• Luster: Spinel has a vitreous (glassy) luster.

• Transparency: It can be transparent to translucent.

Chemical and Physical Properties:

• Chemical Formula: MgAl2O4.

• Hardness: 8 on the Mohs scale.

• Refractive Index: 1.718.

• Specific Gravity: 3.60.

• Cleavage: Spinel has no true cleavage, meaning it doesn't break along specific planes.

• Single Refractive: It is singly refractive, meaning light doesn't split into two rays when passing through it.

Occurrence:

• Spinel is found in various geological settings, including:

  • Igneous rocks: Rocks formed from cooled magma or lava.

  • Metamorphic rocks: Rocks altered by heat and pressure.

  • Contact metamorphic limestone deposits: Limestone altered by the heat of nearby magma.

  • Gravel deposits: Often found alongside corundum (ruby and sapphire).

• Localities: Spinel is found in many locations worldwide, including Myanmar, Sri Lanka, Tanzania, and others.

Historical Significance:

• Spinel has been historically confused with rubies and sapphires due to its similar appearance.

• Some famous historical gems, like the "Black Prince's ruby" in the British Crown Jewels, are actually spinel.