Skepticism

Geology Word of the “Week” Triple Feature: Word III- Peridot

Wrapping up this week’s triple feature is one of my favorite geology words: peridot. A peridot is a gem-quality olivine [(Mg,Fe)2SiO4], a beautiful green mineral found in mafic to ultramafic rocks.

My engagement stone is a peridot– my fiance was pleasantly surprised that my favorite gemstone is among the cheaper gemstones. Though far less durable than diamond, I love the brilliant green color of peridot.

Most gemstones have alter ego mineral names. Below are some examples:

Peridot- Olivine: (Mg,Fe)2SiO4

Ruby- Corundum (Red): Al2O3

Sapphire- Corundum (All other colors except red): Al2O3

Moonstone- usually Potassium Feldspar: KAl2Si3O8

Tanzanite- Zoisite (Blue): Ca2Al3(SiO4)(Si2O7)O(OH)

Amethyst- Quartz (Violet): SiO2

Aquamarine- Beryl (Blue/Turquoise): Be3Al2(SiO3)6

Emerald- Beryl (Green): Be3Al2(SiO3)6

These are just a few of the many examples of gems with both gem names and mineral names. Note how some minerals have multiple gem names depending on their color. Makes learning geo lingo a little more difficult, doesn’t it?

To be fair, some of the gem names undoubtedly originated before the mineral types were discovered/invented. Also, while color is usually a poor way to identify a mineral, color is very important for gemstones. Thus, it makes sense that some minerals such as corundum and beryl (which come in many colors) have multiple gem names. Interestingly, diamonds are always diamonds– no matter the color.

Evelyn

Evelyn is a geologist, writer, traveler, and skeptic residing in Cape Town, South Africa with frequent trips back to the US for work. She has two adorable cats; enjoys hiking, rock climbing, and kayaking; and has a very large rock collection. You can follow her on twitter @GeoEvelyn. She also writes a geology blog called Georneys.

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19 Comments

  1. I love emeralds, but they are so crazy expensive.

    I work with a lot of gemstones with my jewelry making (mostly semi-precious) and the woo surrounding gemstones never ceases to amaze me. Every stone has all these magic powers and people take them so seriously that sometimes get really frustrating when shopping for them.

  2. How do you pronounce “peridot”? I’ve been wondering that ever since they added such items to World of Warcraft. I’ve been pronouncing it (in my head) as “pear-ih-doh” since it looks kinda French.

  3. That is a really pretty mineral! I love minerals. I’ve always been more interested in them than gemstones. I’ve never been much of a diamond fan, too commercial for me. I like unique stuff. The more unique the better. Red, green, and black colored minerals are my favorite.

  4. @Bjornar: thanks! Spelling error noted.

    @Bevans: I pronounce the t in peridot. That’s the way I’ve always heard geologists (at least American ones) pronounce it.

    @Everyone: I guess my next word of the week will be ultramafic. I forget how much geo lingo has slipped into my daily conversation. I didn’t even think of explaining the word. It’s in the works…

  5. Ah, Mineralogy. Evelyn – wanna take out the pear wood crystallography models and play blocks? (Or was that only my sadistic professor?)

    I do love peridot – that apple green color is just so unusual in gems, even considering the range of colors in minerals. I’m picky though – I haven’t found a piece of peridot jewelry I really love…yet…so I don’t own any.

    I, however, am a big fan of Kunzite – Spodumene LiAl(SiO3)2 . Which was always a surprise to me, because I am not a girl that likes pink or lilac a whole bunch. There’s a hunk of it in a Paloma Picasso designed piece of jewelry at Natural History that’ll knock your socks off. (Not the best picture, the NMNH doesn’t seem to provide one.)

  6. I like Opals, principally because they vary so much between individual stones. They seem different from those you list in that they aren’t or don’t seem to be crystalline. I’ve wondered whether or not they comprise some unique class of gemstone. Good article–thanks.

  7. I’ve always preferred other stones far above diamond -and my husband, too, was thrilled to be able to choose a garnet for my engagement ring and to give me amethysts and (faux) blue topaz.

    I’m pretty much that my favorite earrings are made of peridot, now that I see it in this blog entry. :-)

  8. @kittynh:

    *Note – boring diamond info – skip if you like*

    A lot of the black diamond jewelry on the market today has been artificially heated under high pressure or irradiated. That doesn’t mean there aren’t naturally occurring gem quality stones, but naturally occurring, black diamonds have inclusions/impurities of graphite or iron, and gem quality stones are rare.

    Diamonds that are naturally pure are white, otherwise, most actually have a color of varying degree (hence the “color” of the 4C’s – many diamonds can have a yellow tinge to them, though a good setting and proper cutting can minimize it).

    Diamonds are colors other than white because of chemical impurities, altered crystal structure, natural irradiation (though that usually results in a green diamond) or some combination thereof. (For example, natural blue diamonds have a ton of boron in them.)

    If you’re referring to “carbonados”, though, they are kind of a mystery, and are actually an aggregate of lots of smaller porous diamond crystals and are totally f’d up.

  9. @Noadi:

    I love emeralds, but they are so crazy expensive.

    Lab-grown emeralds are fairly reasonable, and are less prone to the micro-cracks that, in natural emeralds, are temporarily concealed by immersing the stone in a hot, pressurized oil bath.

    Um, looking at your webpage, I guess you already knew that. Here’s a good Skepchickal question, though:

    If you are buying a gemstone faceted, instead of in its natural form, is there any reason to prefer a natural stone over a lab-grown one, especially if the lab-grown one is some combination of less expensive, larger, or better quality?

    I like to collect good examples of naturally occuring crystal habits other than what new-agers think a “crystal” looks like. Like:
    http://media-2.web.britannica.com/eb-media/02/2702-004-4F8E8021.jpg

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