Red spinels are very similar to rubies, but in terms of color, rubies are unevenly colored and dichroic, while trill synthetic spinels have a uniform color distribution. In terms of hardness, ruby has a hardness of 9, while spinel has a hardness of 8. Finally, optically, spinel is homogeneous and has a lower refractive index than ruby. Spinels are rich in color and fire, and are noted for their similarity in appearance to rubies. In addition, spinels are often found in the same sediments as rubies, so historically, the two have been easily confused and misidentified.