It is a magnesium aluminate. Spinel often grows in the same deposits as rubies, which is why they were confused for a long time.
Like corundum, spinel has a good hardness, 8 on the Mohs scale.
In addition to Myanmar (formerly Burma), which is well known for its orange-red varieties, spinel is mined in Sri Lanka, Thailand, Afghanistan, Brazil, Turkey, the United States and Nigeria.
Many of the "rubies" found on royal crowns are actually spinel.
One of the most fascinating gems in the world is the "Timur Ruby", a 361-carat red spinel cut into a cabochon with the names of various owners engraved on it. This stone is currently owned by Queen Elizabeth of England.