August Birthstone: Perfect Imperfections Give Rise to Spinel's Striking Colors

John Legend famously sang about his stunning wife Chrissy Teigen's "perfect imperfections" in his 2013 hit, "All of Me." In the world of gemstones, perfect imperfections can turn a nondescript clear stone into a vibrant masterpiece. Such is the case with spinel, the August birthstone that is colorless in its pure state, but brilliantly colorful when impurities are sprinkled by nature into its chemical makeup.

Spinel is a magnesium aluminum oxide that forms when impure limestone is altered by heat and pressure. The mineral gets its color from trace amounts of metallic elements, such as chromium, iron and cobalt.

When chromium replaces aluminum in the chemical structure, for example, the result is a red spinel. When iron or cobalt enters the mix, the result is blue or greenish blue. The other colors represented in the spinel family include peach, pink and purple.

The suite of round brilliant-cut spinels seen 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.

Spinel has the distinction of being the newest official birthstone. In 2016, the American Gem Trade Association and Jewelers of America announced that spinel would be joining peridot as an official birthstone for the month of August. Since peridot is available in only one color, spinel provided more variety and options for August-born individuals.

Spinel is often called a "ruby imposter" because throughout history gem experts had a really tough time telling them apart. In nature, spinel and ruby are often found side by side.

In fact, some of the world’s most famous “rubies” are actually vivid 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.

The Smithsonian noted that the leading sources of spinel are Myanmar, Sri Lanka and Thailand, while other significant spinel production is taking place in Cambodia, Pakistan, Afghanistan, Tajikistan, Tanzania, Vietnam, Russia and the US.

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: Photo by Ken Larsen / Smithsonian.