Garnet is the birthstone for January, but there really isn’t a single mineral named ‘garnet.’ It’s a group of at least 15 minerals of differing chemistry, but only six are common. They are three aluminosilicates combined with iron, magnesium, or manganese, and three calcium silicates in combination with either iron, aluminum, or chromium. The definition of a mineral is something with a specific chemistry and a definite crystalline structure.
All six garnet minerals have essentially identical crystal structure. They are in the isometric or cubic crystal system, meaning their three crystal axes are perpendicular and the same length – that’s what isometric means, same length. But garnets seldom form cubes. The molecules are of sizes that result in many-sided forms, including dodecahedrons, with 12 faces, and trapezohedrons with 24 faces, and more complicated shapes, all of which tend to make garnets almost spherical. Often, they look like little soccer balls in the rock.
The word garnet is from an early English word that meant dark red, as many garnets are, but it may ultimately be derived from the words pomum granatum, Latin for pomegranate – because pomegranates have bright red seed covers similar in appearance to common garnets.
And they’re pretty. Red, purple, yellow, brown, orange, bright green for uvarovite, the chromium garnet – and they’re hard, usually around 7 or higher on the Mohs hardness scale. This makes them great for two things: gemstones and abrasives, and they’ve been used for both since the Bronze age.
By far the greatest use of garnets, by volume, is as abrasives. The US uses about 190,000 tons of garnets every year, with about 17% of that mined in New York’s Adirondack Mountains and in Idaho. The rest, 83% in 2014, was imported mostly from Australia and India and a bit from China. India produces about half the world’s industrial garnet, with China second with a third and Australia 15%. The US is a distant fourth with only 2% of world production.
You benefit from garnets every day, even if not directly in the rough surface of an emery board or sandpaper. Waterjet cutting is used to cut and shape metals from steel to aluminum as well as plastics and glass. Besides their use as abrasives, about a fifth of garnet consumption in the United States goes to filtration in water wells.
The value of industrial garnet in the US is about $9 million. Garnets are used for gemstones, but because they are so common, their value is nothing like that of diamonds, sapphires, rubies, and others, most of which are also harder than garnet and much rarer. But garnets can still make beautiful jewelry.
Do I have any garnets in my collection? Yup. Hundreds of thousands of them, but most of them are microscopic, in a rock with a blue mineral called glaucophane from the California Coast Ranges. The biggest one I have is maybe an inch or so across, from Ontario, but at Gore Mountain, New York, there are garnets as much as two feet across in what is possibly the largest garnet deposit in the world. They are in metamorphic rocks related to a continental collision about a billion years ago, called the Grenville Orogeny.
So happy birthday to everyone with a January birthday. Have a garnetiferous day!
—Richard I. Gibson