This week’s post will cross over into the sphere of gemstones and jewels. Things may get weird, things may get a little bit confusing….but that’s truly what makes the differences between the gemological and the geological so fascinating.
A couple of months ago I found myself somewhere I never thought I would be: in New York City’s “Diamond District”. Coming out of the subway station, I walked past shop after shop with window displays of diamonds and other jewels. I was there to talk to a gemologist, eager to ask all the questions I could about her extensive knowledge of gemstones.
Gemstones vs. minerals
So what are gemstones? And how are they different than what we know (in the geological world) as minerals? Gemstones are (mostly) minerals that are cut and polished specifically for use in jewelry and other types of ornamentation. I say “mostly” here because there are gemstones, like pearls or amber, that are not technically minerals1. Pearls, of course, are made in the shells of mollusks like oysters. Amber is fossilized tree resin. Previously, gemstones were divided into the “precious” group: diamonds, emeralds, rubies, and sapphires versus the “semi-precious” group: everything else. However, “precious…ness” is in the eye of the beholder, and these terms aren’t really used any more as some “semi-precious” gemstones can be more valuable than “precious” ones1. The major difference between gemstones and the minerals that we geologists study is the fact that gemstones must have very specific qualities that make them aesthetically valuable. Let’s go back to our visit to the Diamond District to explore this a little further.
While visiting with the gemologist, I watched her expertly analyze the quality of some diamonds. She carefully notated the “4 C’s” of each diamond: the cut, carat (or weight), clarity, and color. She also made note of the “fire” and “brilliance” of each diamond. The fire is a determination of how well the diamond breaks light traveling through it into its component wavelengths, forming little sparkles of rainbow-colors as you move it. The brilliance of a diamond is a reflection (literally!) of how light bounces off the interior of the diamond. Finally, she notated the “luster” of each diamond2. Geology students will recognize that one as an observation we commonly make of mineral samples – how much light reflects off the surface of the mineral (rather than the interior of the mineral like with “brilliance”). Clearly, there are many, many indicators of what makes a diamond highly valued. And the really interesting part about that is the fact that an ideal diamond highly prized for its perfection as a gemstone would not be nearly as interesting to a geologist.
To think about this, let’s picture the “perfect” diamond gemstone based on the figure above. Internally flawless. Colorless. What do these mean? Both of these “ideal” conditions means the diamond doesn’t have much else in it besides plain old carbon atoms surrounded by other carbon atoms. When deciding the “clarity” of a diamond, gemologists will look for what we call “inclusions”. These are typically blebs of liquid or even other minerals that are encased in the diamond. A colorless diamond is also missing any trace amounts of other elements (nitrogen in the case of the yellow-ness of the diamond) that might change the hue of the diamond itself.
We geologists love to find minerals that would make really lousy gemstones. That’s because additions to a mineral like an inclusion give us a wonderful clue to the origin and formation history of that mineral. For example, volcanic minerals with inclusions are literal pressure capsules of the magma composition when that mineral was formed. Without mineral inclusions, we would only really know the composition of the magma when it reaches the surface, giving us an important clue to how magmas evolve in their composition prior to an eruption. Inclusions in diamonds are used in the geological world to analyze the pressure, temperature, and compositional setting where that diamond formed. This is important because diamonds form very, very deep (up to 90 miles or more!) beneath the Earth’s surface, at extremely high pressures (up to 50,000 times higher than the pressures we experience here on Earth’s surface) and temperatures (over 2,000 degrees Fahrenheit or 1050 degrees Celsius)4.
What’s in a name?
The image below shows birthstones for all the months from January – December, each with a specific gemstone5. If you were born in November, your birthstone is topaz. If you were born in March – aquamarine. May? You get emerald. Seems pretty straight forward, right? The absolutely fascinating piece to this story is that many of these gemstone names have completely different names in the geological world!
For example, the two different gemstones “emerald” and “aquamarine” are actually just the same geological mineral – beryl. Aquamarine is beryl with a bit more iron in it while emerald is beryl with a bit more chromium in it. In an undergraduate mineralogy class (where geologists in training first learn about how to identify minerals), we don’t necessarily learn the gemstone names. This was obvious to me a couple weeks ago when I was visiting an elementary school classroom as part of the Skype a Scientist program (check it out at https://www.skypeascientist.com/). A student asked me if I knew anything about the gemstone ruby. Of course, I had heard of rubies before….but which mineral is ruby? Ruby….ruby….oh! Corundum. In fact, rubies and sapphires are both corundum….just red in color with more chromium (ruby) or….anything but red and added bits of titanium or iron (sapphires). Strangely enough, there are some birthstone names that are not quite as creative. October has tourmaline, which is an amazing mineral that can be a wide variety of colors. January has garnet, which is a beloved mineral with many different varieties making up our second post here at Backyard Geology.
No really…what’s in a name?
We’ve talked a bit before about differences in naming between geology and industry. For example, Jess told us about granite countertops that are sometimes not even close to being what we in geology would call “granite”. Well, gemstone names give us a couple more examples of pretty wild naming schemes. For example, the picture below was taken in a mineralogy classroom here at Columbia University. You can see the actual mineral names of the specimens (Quartz, with a mineral formula of SiO2) and then the gemstone names in parenthesis. You’ll notice that quartz can have many different gemstone names such as “citrine” (yes, that crazy magical crystal featured in Wonder Woman 1984) and….”Spanish Topaz”. Wait. Spanish Topaz??? The problem here is that topaz is a completely different mineral than quartz, with a mineral formula of Al2SiO4(OH)2. In the gemological world, topaz (the November birthstone) is a gemstone that is often yellow in color…so sometimes yellow quartz gets the name “Spanish Topaz”.
Finally, I think the best example of oddly named gemstones is the “Black Prince’s Ruby”, a 34 gram (170 carat) red gemstone seated in the middle of the Imperial State Crown currently worn by Queen Elizabeth II of the United Kingdom. If this gemstone is a ruby then it must be beryl, with a mineral formula of Be3Al2Si6O18…right? Nope! In the late 1700’s it was discovered that this mineral was actually spinel, with a mineral formula of MgAl2O4. So we call this the “Black Prince’s Ruby” but really it should be the “Black Prince’s Ruby, um, Spinel”6.
So the next time you find yourself face to face with a gemstone, consider the forces (both geological and societal!) that have come together to create these highly prized jewels.
1 Gemstone Encyclopedia, International Gem Society, https://www.gemsociety.org/gemstone-encyclopedia/, accessed 3 December, 2021.
2 Devouard, B. and Notari, F., 2009. The identification of faceted gemstones: From the naked eye to laboratory techniques. Elements, 5(3), pp.163-168.
3 Logvinova, A.M., Taylor, L.A., Floss, C. and Sobolev, N.V., 2005. Geochemistry of multiple diamond inclusions of harzburgitic garnets as examined in situ. International Geology Review, 47(12), pp.1223-1233.
4 How Diamonds are Formed, Cape Town Diamond Museum, https://www.capetowndiamondmuseum.org/about-diamonds/formation-of-diamonds/
5 Birthstones. International Gem Society, https://www.gemsociety.org/article/birthstone-chart/, accessed 3 December, 2021.
6 Beckett, K. “Spinel: ‘The Great Impostor’ No More” New York Times, 10 February, 2020, https://www.nytimes.com/2020/02/10/fashion/jewelry-spinel-christies-sothebys-cartier.html