Spinel — August's newest official birthstone — is famous for being a ruby imposter. This is because throughout most of recorded history, gem "experts" couldn't tell them apart.
In fact, some of the world’s most famous “rubies” are actually red spinels. These include the 170-carat Black Prince Ruby, which is prominently displayed on the Imperial State Crown of England; the 361-carat Timur Ruby, which was presented by the East India Company to Queen Victoria as a gift in 1851; and the 398-carat ruby look-alike that tops the Imperial Crown of Russia. Catherine the Great commissioned the crown in 1763 and never knew that her impressive "ruby" was actually… a spinel.
According to the Smithsonian, it wasn’t until 1783 that spinel was recognized as a mineral distinctly different than corundum (ruby and sapphire). Ruby is composed of aluminum oxide, while spinel is made of magnesium aluminum oxide. Both get their reddish color from impurities of chromium in their chemical structure. In nature, they are often found side by side.
The spinels, above, are from the Smithsonian's National Gem Collection. The 22.20-carat rosy pink spinel is from Sri Lanka and the 36.10-carat red spinel is from Burma.
In 2016, the American Gem Trade Association (AGTA) and Jewelers of America (JA) announced that spinel would be joining peridot as an official birthstone for the month of August. The news came as a surprise to jewelry aficionados because the modern birthstone list had been amended only three times over the span of more than a century.
Established in 1912 by the American National Retail Jewelers Association (now known as JA), the modern birthstone list was updated in 1952 to add alexandrite (June), citrine (November), tourmaline (October) and zircon (December). The list was amended again in 2002 when tanzanite joined the group of December birthstones.
While August's newly designated official birthstone is widely recognized in its deep red variety, spinel is also available in a wide range of colors. Pure spinel is colorless, but impurities give rise to a spectrum of choices, from peach, pink and red to green, blue and purple.
The suite of round brilliant-cut spinels in the photo, above, has been part of the Smithsonian's National Gem Collection since 2013. The Vietnam-sourced gems range in size from 60 points to 4.60 carats. The Smithsonian noted that the leading sources of spinel are Burma, Sri Lanka and Thailand, while other significant spinel production is taking place in Pakistan, Afghanistan, Tajikistan, Tanzania, Vietnam and Russia.
Spinel is a durable gem with a hardness of 8.0 on the Mohs scale. By comparison, diamond rates a 10 and ruby rates a 9.
Credit: Red spinels by D. Penland/Smithsonian. Spinel suite by Greg Polley/Smithsonian.