Spinel is a hard vitreous magnesium aluminum oxide that has been used as a gemstone for centuries.Spinel is often assumed to be other gemstones, such as ruby and sapphire. However, spinel can be distinguished from other gemstones by its octahedral crystal structure and single refraction. Additionally, spinel has a lower Mohs hardness than ruby and sapphire.The rarest and most desirable spinel gemstones are vivid ruby-like red, followed by cobalt blue, bright pink and bright orange. Paler colors such as lavender tend to be more affordable. Other popular colors include black spinel, violet-blue, greenish-blue, grayish, mauve, yellow, brown and pink spinel.Good quality spinel gemstones should have no visible inclusions. Inclusions typically decrease the value of spinel gemstones. However,desirable inclusions can result in asterism (the star effect). Inclusions in spinel can resemble its octahedral crystal structure or a human fingerprint. Spinel has a vitreous luster.