Spinel

Spinel is a mineral composed chiefly of magnesium aluminate (MgAl₂O₄). Its crystal structure and trace impurities produce a wide range of colors, including red and blue varieties that can resemble ruby or sapphire.

Spinel group: A mineral group sharing the spinel crystal structure, including minerals with varied chemical compositions. Spinel is the group’s magnesium-aluminum member.
Crystal lattice: A repeating three-dimensional arrangement of atoms, ions, or molecules in a crystalline solid. Spinel’s ordered lattice governs its structure and the sites impurities can occupy.
Ruby: Red gem-quality corundum, colored mainly by chromium. Red spinel was historically mistaken for ruby despite its different mineral composition.
Gemstone: A mineral or other material cut and polished for ornamental use. Spinel’s hardness, luster, and color make it a durable gemstone.
Georgius Agricola: A sixteenth-century German scholar whose writings helped establish mineralogy and mining as systematic subjects. His mineralogical work helped document distinctions among minerals known as spinel.
Magnesium aluminate: An inorganic compound with the chemical formula MgAl₂O₄. This compound is the principal chemical constituent of mineral spinel.
Cation inversion: A redistribution of cations between different crystallographic sites in a crystal structure. The degree of inversion affects spinel’s physical and spectroscopic properties.
Sapphire: Gem-quality corundum of colors other than red, commonly blue. Blue spinel can resemble sapphire but belongs to a different mineral species.
Balas ruby: An old trade name for pale red to rose-colored spinel. This historical variety illustrates spinel’s former confusion with ruby.
Pliny the Elder: A Roman author whose Natural History compiled ancient knowledge of nature and minerals. Ancient gem descriptions contributed to the long-standing confusion between spinel and ruby.

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