Garnet is a familiar gemstone. Most lovers of jewelry would recognize its deep, rich red hue, reminiscent of a pomegranate seed, the source of garnet’s name. But many don’t realize that garnet, in fact, refers to a group of minerals that come in a rainbow of colors! Red is the most common color and occurs in several of the species of January’s birthstone, but the whole spectrum is represented – through warm oranges, rich golden yellows, lush greens, rare blues, and royal purples.
The garnet group consists of six major mineral species (and a whole bunch of minor ones not used as gemstones). These six species are divided into two solid-solution series: the aluminum garnets, sometimes called the pyralspite series, and the calcium garnets, referred to as the ugrandite series.
A solid-solution series of minerals essentially means that the mineral species can “blur” into one another: in pyralspite garnets, for example, the chemical composition is x3Al2(SiO4)3, with x replaced by iron, magnesium, or manganese. When a garnet with this composition is very rich in iron, but has little magnesium or manganese, it’s considered an almandine garnet. But if you were to slowly remove iron and replace it with magnesium, you’d be left with a pyrope garnet, a whole other species! The boundary between these two examples is blurry, and there’s an intermediate “pyrope-almandine” where iron and magnesium are both present.
This familiar red hue is characteristic of pyrope and almandine garnets.
But what are pyrope and almandine, anyway? Pyrope is the most common garnet species: what most people think of when they picture garnet. It’s dark red in color with a glassy surface luster. Almandine, on the other hand, has a higher luster – bordering on diamond-like – and always has some red in its coloring due to the iron present. Garnets intermediate these, in the pyrope-almandine range, can come in a rich purple-red hue, these stones being known as rhodolite. The final member of the pyralspites is called spessartine, the manganese garnet. Manganese adds orange coloring to these garnets, so when a garnet is pure spessartine – that is, there’s no iron or magnesium present – it’s bright tangerine-orange. Those with more iron can appear a dark orangey-red color. Interestingly, an intermediate between pyrope and spessartine (you guessed it: pyrope-spessartine garnets) can come in bright blue and even exhibit a color change effect similar to the rare and valuable alexandrite! These garnets originate in Madagascar and are the only truly blue garnets available.
You may now be wondering what the other half of the garnets are, since the first series colored such a large color range! But as pyralspites cover the red-purple-blue side of the spectrum, the ugrandites – the calcium garnets, with a chemical structure of Ca3y2(SiO4)3 – come in every shade of orange, yellow, and green. The y in the ugrandites’ chemical composition may be replaced with iron, aluminum, or chromium, each resulting in a different species.
Let’s start small: the uvarovite species of garnet, with chromium in its composition, forms tiny, vibrant green crystals, almost always too small to be faceted, but making for a striking, glittery centerpiece to a piece of jewelry when the crystals blanket a piece of host rock. When the chromium is replaced with aluminum, the result is grossular. Perhaps the garnet with the largest color range, grossular ranges from deep scotch-browns through lighter oranges all the way to emerald green! The warm, cinnamon-colored gemstones are called hessonite in the jewelry trade, and the vibrant green stones are known as tsavorite.
This demantoid garnet contains a stunning, firework-like horsetail inclusion. Photo from Gemcamp Laboratories.
And finally, when iron is present in large enough amounts, the result is andradite, a garnet with sparkle that rivals diamond! Too much iron results in the stone becoming opaque and black (a variety known as melanite), so a delicate balance between iron and other elements must be achieved to create the bright hues and glittering, diamond-like fire possessed by andradite garnets. Chromium, making up only 0.014% of the Earth’s crust, sometimes finds its way into the already rare andradite garnets, creating a rich green hue and resulting in the rarest of the garnet gemstones, known as demantoid. These beautiful gems come primarily from the Ural Mountains in Russia, the only known source until the 1990s! Demantoid is also known for having dramatic sprays of mineral inclusions known as horsetails, which can increase the value of the stone significantly. These horsetail inclusions only occur in demantoids from Russia, making it the most important and sought-after source for this rare gem.
Though many other garnet species exist – even a few that aren’t found in nature and have only ever been produced in laboratories, such as the tongue-twisting gadolinium gallium garnet – those outlined above are the main natural garnet species used as gemstones today. That means January has the most birthstones of any month, with a total of six unique species! What a colorful way to start off the new year!
