Traditionally, spinel has not been treated because it was born with natural good looks. As it has become more scarce and demand has increased, more research has taken place on treating non-gem grade spinel. Mostly involving heat treatments, there is little success to date, most often resulting in a less attractive stone than before. So for now, it is still safe to assume spinel is not treated, but this may not always be the case.
Spinel was one of the first gemstones to be synthesized by the flame fusion method at the end of the 19th century. According to legend, it formed as a mistake while trying to synthesize ruby and sapphire. As it could be grown in any color, was hard and brilliant, it became the ideal gem material used for imitation birthstones and class rings. For years Spinel suffered under the assumption it was a synthetic gem, because the natural version was so rare while the synthetic was literally everywhere. In the 1980's the first flux grown synthetics appeared in the market.
Like most stones, a spinel's color generally determines its value. In addition to the intensity and saturation of its color, the gem's hue itself is important in valuing spinel, because certain spinel hues are more valuable than others due to their rarity. Pure red or slightly purplish-red spinel with medium to medium-dark tone is typically the most in-demand and valuable color of spinel, followed closely by cobalt blues (especially those from Vietnam, Sri Lanka, Madagascar, and Pakistan), as well as almost neon hot pinks and flaming oranges (especially those from Burma/Myanmar). Finest spinel blues are said to be of medium to medium-dark saturation and resembling fine blue sapphires; the most desired neon pinks and hot oranges are so fluorescent that it affects even their "standard" appearance, making them appear to glow even in broad daylight. Spinel in these colors over five carats are scarcely available and priced accordingly. More recently, pastel hues have become more popular, especially the blues and lavenders from Vietnam. Easy-going spinel can usually be safely cleaned in ultrasonic or steam cleaners, or with simple warm soapy water. Spinel's good hardness and toughness make it an ideal stone for jewelry.