Some of the rarest gems in the world are spinels, even if many of them were once believed to be 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. But in fact all natural spinel are fairly rare, and that's 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, created using the flame fusion process, actually 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 the near-equal of ruby and sapphire. Spinel is a magnesium aluminate, typically colored by chromium and iron and, occasionally, cobalt. It is very hard (8 on the Mohs scale, compared to 9 for ruby and sapphire), and it forms as a cubic crystal like a diamond. Spinel occurs in octahedral crystals but fortunately has very poor cleavage (unlike diamond, which has perfect cleavage). Due to spinel's very good dispersion, gem spinels can possess vivid fire, and the intensity of spinel color is in part due to the fact that spinel is one of the few singly refractive gemstones (the others being garnet and diamond).
Almost all the ruby and sapphire in the market has been heat treated to improve color and clarity. But spinel is a gem which is never treated. That makes spinel an unusual gem in today's market.
Burma and Sri Lanka are the traditional sources for fine spinel, though the Burmese material is generally recognized as superior. But the supply is very limited and is not sufficient to supply the retail jewelry trade. So many jewelers simply don't stock spinel and thus the market doesn't promote it. This can make natural spinel quite difficult to find. But it also means that prices are surprisingly reasonable. Compare the prices for unheated ruby and sapphire (if you find them)!
Spinel is judged by color and clarity, the same as other colored gemstones. The finest spinel have a saturated pure color, with minimal brownish or gray overtones. The red and pink spinel are the most prized, followed by the lavender and blues. Any spinel over 2 carats is rare, and good stones over 3 carats count as very rare.
Recently some high quality African spinel has come on the market, with some pieces in good sizes. We have found some excellent spinel from Tanzania in red, pink, violet, and blue; and occasionally in unusual colors such as orange and purple. The supply of Burmese spinel is very limited but steady, and the material is generally of very high quality.