If you read the description of ruby, you’ll know that in the old days sometimes spinel was mistaken for ruby. It’s not surprising, since spinel can be the same color, is almost as hard as ruby, has almost the same “refractive index” (it reflects light just as well), and has a pretty similar formula, just with some magnesium (Mg) added. It even forms in the same kinds of places as ruby. Its name is based on the Latin word for “spine”—not like the spinal column in your back, but like something pointy—because it can have pointy ends on its crystals. Similar to sapphire, spinel can also occur in purple, blue, green, pink, or orange. Unlike sapphire, though, spinel often forms eight-sided crystals–kind of like eight-sided dice instead of six–whereas the classic shape for natural sapphire crystals looks like a long, stretched-out triangle.