The Rare Red Stone of the British Crown Jewels
Spinel is in itself a mineral and is composed of magnesium aluminum oxide. It can occur in a wide range of color - pink to a rich red, lavender to deep violet, light to deep blue, orange, yellow, brown, and black.
The red variety has the same coloring agent as ruby, chromium. This element causes both stones to glow a vibrant red underneath ultraviolet light, making them difficult to discern apart. The difference is spinel is singly refractive and will not display pleochroism like doubly refractive corundum.
Spinel grows in the same deposits as ruby and sapphire with significant quantities in Cambodia, Myanmar, Sri Lanka, and Thailand with smaller deposits in the US, Brazil, parts of Africa, and Australia.
Spinel's name is derived from Latin's 'spina' to mean "thorn". This is because well-formed crystals grow as a pointed octahedron, much like a diamond. As you're probably noticing, spinel is clever and often seems like other gemstones if not properly accessed.
Before gemological testing and even before the field of mineralogy was established, spinel was being mined and fashioned into incredible pieces of jewelry.
Specifically the British Crown Jewels. Arguably the most famous spinel is misnamed as the Black Prince's Ruby and is an incredibly saturated candy apple red cabochon front and center of the Imperial State Crown. This gem along with the Timur Ruby have been treasures of British royalty for centuries when finally in 1783 mineralogists determined spinel to be a different mineral than corundum.
In Jewelry and Care
Spinel is a hard and durable gem which makes it perfect for everyday jewelry wear. Spinel is often faceted to display its color but older specimens were typically cut as cabochons.
The gemstone has recently been added as an alternate birthstone for August.
Spinel can be soaked in an ultrasonic cleaner by a qualified jewelry professional or it can be washed with warm soapy water and a soft bristled brush. When storing be sure to wrap spinel to prevent it from scratching softer gemstones.