Some of the rarest gems in the world are spinels and their fineness caused many of them to be once mistaken for rubies or sapphires. For example, the enormous Black Prince's Ruby that adorns the Imperial State Crown of the United Kingdom is actually a spinel. All natural spinel gems are fairly rare and that is why they can be difficult to find in jewelry stores. It is also, paradoxically, why the prices for spinel are still reasonable.

When many people think of spinel, they think of the cheap synthetic spinel used in low-end birthstone jewelry and class rings. This synthetic spinel that is created by the flame fusion process has a different chemical composition from natural spinel, and thus a different refractive index and specific gravity. Synthetic ruby, on the other hand, can be difficult to distinguish from natural ruby.

Natural spinel is a very fine gemstone indeed, with many characteristics that make it almost equal to ruby and sapphire. Spinel is composed of magnesium aluminate and is typically colored by chromium and iron. Occasionally, traces of cobalt are the coloring agent. It is very hard (8 on the Mohs scale, compared to 9 for ruby and sapphire) and it forms as a cubic crystal like diamond. Spinel occurs as octahedral crystals and fortunately has very poor cleavage (unlike diamond, which has perfect cleavage), which makes it very durable. Due to its very good dispersion, spinel can possess vivid fire; the intensity of color is in part due to the fact that spinel is one of the few singly refractive gemstones (the best-known others being garnet and diamond).

Almost all the ruby and sapphire available for sale has been heat treated to improve color and clarity. However, spinel is a gem that is never treated. This makes spinel an unusual gem in today's gemstone industry.

Spinel is judged by color and clarity, the same qualities as other colored gemstones. The finest spinel gemstones have highly-saturated, pure color with minimal brownish or gray overtones. Red and pink spinel gems are the most highly-prized, followed by lavender and blue stones. Any spinel gem weighing over two carats is rare and good stones weighing over three carats are extremely rare.

Burma and Sri Lanka are the traditional sources for fine spinel and the Burmese material is generally recognized as superior. However, the supply is limited and insufficient for the retail jewelry trade. Therefore, many jewelers simply do not stock spinel and as a result the market does not promote it. This can make natural spinel quite difficult to find, but it also means that prices are surprisingly reasonable, especially when compared with the prices of unheated ruby and sapphire.

Recently, some high quality African spinel has appeared on the market, with some pieces in good sizes. Red, pink, violet, blue, and occasionally, unusual orange and purple spinel from Tanzania is now available. The supply of Burmese spinel continues to be very limited but steady, and the material is generally of very high quality.