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.

 

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.

 

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!

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“This one is 1.3 carats,” the saleswoman explained, showing a loose diamond in a small, folded paper, “but it’s internally flawless!” The couple hadn’t seen an internally flawless stone yet, and they were intrigued.  Liam thought the stone would suit Olivia wonderfully. But looking at the price tag, Olivia was skeptical, thinking, I don’t see any difference between this diamond and the others. Why should we pay so much more for it?

Diamonds require very specific conditions for their formation: extremely high pressures (beneath 90 miles of dense rock or places where the Earth’s plates are forced together) and blistering temperatures (usually roughly 2000°F). While they normally aren’t made from coal – in fact, most diamonds formed before land plants existed! – they do require lots of carbon in their environment, since that’s what they’re composed of. As the diamond grows, these conditions must be kept constant. Any change in pressure, temperature, or concentration of carbon can cause interruptions in growth, or even the crystallization of other minerals inside the newborn diamond. And since nature isn’t a carefully monitored pressure cooker, almost every diamond has some kind of internal feature formed during the growth process!

The highest clarity possible in a diamond is called flawless, denoted by the symbol F or FL, followed by internally flawless, with the symbol IF. A flawless diamond must have grown evenly so as to not form any visible irregularities under 10x magnification, the standard at which diamonds are graded. The difference between flawless and internally flawless comes down to the diamond cutter and the stone’s handling afterward: one single scratch, nick, or even a tiny area of lesser polish will cause a once-flawless diamond to receive an IF grade. If a flawless diamond is set into jewelry, in almost all cases (the exception being very important stones) the diamond will be considered only internally flawless, since the setting process could have caused one of these tiny blemishes!

Diamond clarity plots, from GIA.edu.

The grades below IF gradually become more and more included, and therefore more and more common. VVS grades will only have minute inclusions – tiny crystals called “pinpoints”, nearly indistinguishable even with a jeweler’s loupe. VVS stands for “very very slightly included”. The next pair of grades, the VS (you guessed it – “very slightly included”) grades, can have minor inclusions. Tiny crystals or cracks (called feathers) can be present, but are usually positioned toward the stone’s edges. Below VS are the SI (“slightly included”) grades, which can have larger crystals, feathers, or a combination of these features. You’ll also commonly see inclusions caused by changes in the diamond’s growth direction, called twinning wisps. These inclusions may be positioned towards the center of the diamond, and will be quite noticeable to a trained professional under magnification.

The next grades are the I (“included”) grades, of which there are three. I-1 and I-2 diamonds have large inclusions, usually feathers, positioned in a way that they are very obvious under magnification. I-3 diamonds are full of inclusions, often to the point that the stone no longer appears transparent! All three I grades can affect the appearance of the diamond to the naked eye, and even an untrained observer may notice the inclusions in an I-2 or I-3 diamond. Below the I grades are severely included diamonds, which for a long time were simply used for industrial purposes, but have recently received a surge in popularity for their unique, rustic appearance. You may hear these stones referred to as “industrial diamonds”, “salt-and-pepper diamonds”, or even “interstate diamonds”! (Okay, fine, that last one is a joke. Think I-5, I-15… You get the idea.) Industrial quality diamonds are very common, especially in small sizes.

Olivia and Liam were shown a few more diamonds at the jewelry store, and narrowed their choices down to two: the internally flawless diamond from earlier, and an SI-1 diamond that was much less expensive. Side-by-side, they looked the same; it was only when the saleswoman showed the two stones in the microscope that Olivia was able to see a difference. How should they choose? Clarity is all about personal preference! The couple had a relatively tight budget, so the SI-1 diamond was a better fit for them (despite Liam’s protests). But another customer, a collector of rarities of all kinds, thought the internally flawless diamond would make a perfect pendant!

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The salesman hadn’t accidentally switched the tags. In fact, the diamonds were inscribed with numbers that had also been printed onto their tags – meaning a quick glance in the microscope confirmed the salesman was correct.

Carat weight is the most apparent of the four diamond quality factors (called the “4 Cs”) to most customers, and for good reason – the size of the diamond is by far the easiest to see! But in truth, carat weight is just that: a measurement of the stone’s weight. One carat is equivalent to one-fifth of a gram, and is measured on a very sensitive scale, usually to the thousandth or even ten-thousandth of a carat. The carat weight is then presented with two decimal places, called “points”; a 0.70 carat diamond can also be called a seventy point diamond. It’s easiest to think of carats like dollars, and points like cents.

Normally, the weight of a diamond does correlate with size. A one-carat round diamond can be estimated to have a diameter of 6.5mm on average, which, compared to the average ring finger width of 15mm, is bigger than many expect! When a diamond cannot be weighed, basic “carat to millimeter” conversion charts are often used to estimate the carat weight based on the diameter of a particular diamond, or vice versa. But as shown in Olivia’s case, these charts aren’t always reliable. Sometimes, a diamond can have a larger or smaller diameter than its carat weight would indicate. The reason is simple: diamonds are three-dimensional!

The depth of a diamond can have a drastic effect on its carat weight without affecting its apparent size. Picture a cube. If you take a slice off the bottom, it’ll still look the same from above, but weigh less; similarly, if you add a section to the bottom, it’ll weigh more without changing its appearance from the top view. Increasing or decreasing the “bulk” around the girdle (or edge) of the diamond will change its weight in the same way, as will changing the angles of the top and bottom (crown and pavilion respectively). This means the most accurate way to estimate carat weight involves measuring the depth of the diamond, and using this measurement along with the average diameter to calculate what the weight should be. This method is usually fairly accurate, but, of course, simply weighing the diamond will always be the best way!

So how should Olivia choose between the two diamonds presented to her? All other factors being equal, she should simply choose the size which she prefers on her finger. But such a large difference in carat weight usually indicates one or both of the diamonds has a lower cut grade – a different topic for a different post – so in that case, Olivia might look at the diamonds side-by-side, and decide which looks prettier to her. A very shallow diamond, called “spready” by many people in the jewelry trade, will look larger than its weight may indicate, but could appear grey or lack fire. A deep diamond will look smaller than its weight indicates, and may be dark and lack brilliance. The first diamond OIivia looked at, weighing 1.52 carats, was one of the so-called “spready” stones, and so she ended up preferring the heavier, smaller-looking diamond – but another customer, who wanted a larger-looking stone on a tight budget, found that the shallower diamond suited their needs perfectly!

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Discovered in 1967, tanzanite is one of the newest members of the birthstone family. Its lively blue and violet hues have captivated jewelry designers of the late 20th and early 21st centuries. But this gemstone is said to be a “generational” stone – what does that mean, and why?
The first tanzanite crystal was said to be found by a Maasai tribesman, thought to be Ali Juuyawatu, near the Mererani Hills in Tanzania, the southern foothills of Mount Kilimanjaro. Manuel d’Souza, a local treasure hunter, was alerted to the presence of the stone and, thinking he’d found a new deposit of sapphires, registered several mining claims in the area. The mysterious new gemstone was first correctly identified as zoisite by a geologist for the Tanzanian government.

Hearing of the vibrant new gem, Tiffany & Co. invested in the blue zoisite, dubbing it “tanzanite” after its home country and becoming its primary distributor. The company proudly advertised that tanzanite could only be found “in Tanzania and Tiffany’s”. With its sparkling blue color, tanzanite quickly rose to popularity among designers and consumers alike. In 2002, the American Gem Trade Association dubbed tanzanite a birthstone for December in its first update to the birthstone list since 1912, finalizing its place among historically valued gems like sapphires and emeralds.

These days, many sellers of tanzanite will promote it as an investment stone, explaining that as the only tanzanite mine in the world goes dry, the gem’s prices will skyrocket. But is the world really running out of tanzanite? Geologists estimate the commercial tanzanite mines will be exhausted of the material in 15 to 25 years, making the time from its discovery to its depletion roughly a single human lifespan, thus, a “generational” gemstone. However, in April of 2018, the President of Tanzania, John Magufuli, revealed a new tanzanite source had been discovered, but its location is still under wraps. Additionally, zoisite, the mineral species to which tanzanite belongs, is relatively common worldwide. An entrepreneurial miner need only discover a source of zoisite containing the correct elements to allow it to be heated to a blue color to completely flood the tanzanite market.

Do you think tanzanite will continue to be found after the current mines have run out? The future of this precious gemstone is still up in the air – or rather, down in the earth!

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While diamond is no longer the hardest natural mineral found on Earth (it lost its title to both lonsdaleite and wurtzite boron nitride in 2009), it’s still considered the standard for ultra-hard materials. For most purposes, this means it’s extremely durable, but clearly there’s something else at play here! Don’t worry, you weren’t fooled by a fake: diamonds aren’t actually all that tough.

To understand why your diamond cracked so easily, you must differentiate between hardness and toughness. Hardness is a material’s resistance to scratching, while toughness defines how readily a material will chip or crack (it may be easier to think of it as “brittleness”). Diamonds are very hard, but relatively brittle because of their cleavage. 

So what is cleavage, anyway? No, not that. Cleavage is a property of most minerals caused by planes of weakness in their atomic structure. If you were a rockhound as a kid, you may recall finding glittering sheets of mica and peeling them apart. This is an extreme example of cleavage; the atoms in mica are so weakly bonded along those planes that the mineral splits into sheets, which theoretically could be only an atom thick!

Thankfully, girls’ best friend doesn’t cleave nearly as easily as mica. It takes a pretty hard hit to break a diamond, and it’ll only break if that hit is at just the right angle – the cleavage planes follow the octahedral shape traditionally associated with diamond. Normally, diamonds are cut so that an accidental strike to the girdle, or edge, of the stone won’t damage it. But sometimes a diamond must be cut in a way that exposes a cleavage plane, and a sharp blow, like a long fall to the floor, could be exactly what does your diamond in.

Unfortunately, once your diamond has broken, there’s no fixing it – hopefully you have your jewelry appraised by a GIA Graduate Gemologist Appraiser and insured with a personal article policy! – but the calamity is easy to avoid. Keep your jewelry safely stored in a padded box or pouch, well away from the edges of tall furniture (especially if you have cats), and don’t wear it if you might hit it against something hard. So while it’s tempting to show off your brand new, sparkly engagement ring to your girlfriends at the gym, consider leaving it at home this time and gushing about it over coffee instead!

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