Spinel is a very attractive and historically important gemstone mineral. Its typical red color, although pinker, rivals the color of ruby. In fact, many rubies of notable fame belonging to crown jewel collections were found to actually be spinels. Perhaps the greatest mistake is the Black Prince's Ruby set in the British Imperial State Crown. Whether these mistakes were accidents or clever substitutions of precious rubies for the less valuable spinels by risk taking jewelers, history is unclear. The misidentification is meaningless in terms of the value of these gems for even spinel carries a considerable amount of worth and these stones are priceless based on their history, let alone their carat weight and pedigree.
Today, expensive rubies are still substituted for by spinel in much the same way a diamond is substituted by cubic zirconia. Not to commit a fraud or theft but to prevent one. Spinel may take the place of a ruby that would have been displayed in public by an owner who is insecure about the rubies safety. The spinel probably is still valuable but better to lose a $100,000 dollar spinel than a $1 million dollar ruby!
Spinel and ruby are chemically similar. Spinel is magnesium aluminum oxide and ruby is aluminum oxide. This is probably why the two are similar in a few properties. Not suprisingly, the red coloring agent in both gems is the same element, chromium. Spinel and Ruby also have similar luster (refractive index), density, and hardness. Although ruby is considerably harder (9) than spinel, spinel's hardness (7.5 - 8) still makes it one of the hardest minerals in nature.
Spinel may be the poorer cousin of ruby, but its pinker color and other qualities make it attractive in its own right. Spinel typically forms nicely proportioned octahedrons. But it is famous for a type of twinning that bears its name, the Spinel Twin Law. Spinel Law twinning is also found in other isometric minerals such as diamond, galena, cristobalite, magnetite, franklinite and other members of the spinel group. This type of twinning produces a twin plane that is parallel to one of the octahedral faces. The plane acts as a mirror plane and produces a left and right side that are mirror images of each other. This may not sound all that spectacular for a very symmetrical mineral like spinel which is loaded with mirror planes. However this mirror plane is not parallel to any of the others and actually lowers the symmetry of the crystal (only in appearances though).
A good description of the twin is hard to explain, but here it goes. The plane falls (of course) in the center of the crystal, dividing it in half. The two octahedron faces parallel to the twin plane are equilateral triangles. Each point of the triangles is doubled across the twin plane with an indentation between them. The crystal looks like it has trigonal symmetry, but the three indentations are a clue that this crystal is a twin. Twins of spinel are rare, but their popularity makes them readily available on the market.