Spinel
Spinel is a mineral that owes its beautiful color to chromium, much like rubies and emeralds do.It has a hardness of 8 on the Mohs hardness scale (diamond has a hardness of 10). Spinel is seen in a wide range of colors: red, lavender, violet, blue, green, brown, black. Often, spinel is found in gravel deposits where there is much corundum as well. Spinel typically contains magnesium and aluminum. Spinel is singly refractive, often transparent, with a specific gravity of 3.60 and a refractive index of 1.718. Spinel crystals have no true cleavage.
Well, it wasn’t always easy to tell the difference between a deep red spinel and a ruby. They were often found in the same places, and once polished, the two gems were nearly indistinguishable. And without the scientific equipment we have today, it was much more difficult to differentiate the two gemstones—it wasn’t until 1783 that a mineralogist discovered they were not, in fact, the same thing.