| Red spinel has been historically confused with ruby for many years. The most famous case was the now "famous" Black Prince Ruby that sits a top the British Imperial State Crown. It is now known that this roughly 170 ct. deep red stone is a spinel and not the fabled ruby it was originally called.
Corundum (ruby) is pure aluminum oxide and gets its beautiful red color from eh element chromium. Spinel is a mixture of aluminum oxide and magnesium oxide and the red variety gets it color form the same element, chromium. Two similar chemistries and two stones that are difficult to tell apart. The red stones are sometimes called "balas rubies".
Spinel comes in a variety of other colors as well as the famous red, including, purple, orange, blue, green and black. The non-red varieties get their coloration from iron, vanadium and collat. Large stones are quite rare and stones showing a star are very rare.
Spinel is actually far more rare than ruby, and one would think that it should have higher value, but unfortunately its rareness has actually reduced its value because it has never gained a high popularity. Thus it is less expensive than the equivalent ruby.
Spinel has been synthesized in the laboratory and there is more information in the section on synthetics.
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