Spinel

Spinel is a magnesium aluminum oxide mineral that forms beautiful, brilliant gem crystals in a wide range of colors. Red spinel colored by chromium so closely resembles ruby that several famous historical "rubies," including the Black Prince's Ruby in the British Crown Jewels, are actually spinel.

With a hardness of 8, excellent brilliance, and no cleavage, spinel is both durable and attractive. It is singly refractive, which helps distinguish it from doubly refractive corundum.

Long overshadowed by ruby and sapphire, fine spinel is now recognized and prized in its own right, especially vivid red, hot pink, and electric cobalt-blue stones.

Spinel forms mainly in metamorphic rocks, especially marbles created when limestone is altered by heat and pressure, where magnesium and aluminum recrystallize into octahedral spinel crystals. It also occurs in some igneous rocks and in contact metamorphic zones.

Because it is hard and durable, spinel weathers out of its host rock and concentrates in alluvial gem gravels, often alongside ruby and sapphire. Major sources include Myanmar (Burma), Sri Lanka, Tanzania, Vietnam, Tajikistan (the historic Kuh-i-Lal mines), and Madagascar.

Spinel typically forms sharp octahedral crystals and shows a vitreous luster, hardness of 8, and a white streak. A key diagnostic is that it is singly refractive (isotropic), so it shows no doubling of facets, unlike ruby and sapphire.

It has no cleavage and a relatively high specific gravity (~3.6). Red spinel is distinguished from ruby by single refraction, weaker fluorescence, and lack of pleochroism. Other look-alikes include garnet (different SG and refraction) and glass (softer, with bubbles). Lab gem testing readily confirms it.

Spinel is used as a faceted gemstone in rings, pendants, and earrings, valued for its brilliance, durability, and color range. It was added as an August birthstone in 2016, alongside peridot.

Fine red and cobalt-blue spinels can command high prices, though spinel generally offers excellent value compared with ruby and sapphire. Octahedral crystals are also popular with mineral collectors. Synthetic spinel is widely produced for inexpensive jewelry and simulants. Metaphysically, spinel is associated with energy, renewal, and encouragement. Its toughness makes it well suited to everyday jewelry.