Geology Word of the Week: P is for Peridot

Before I started a geology blog, I used to sporadically blog about geology over at Skepchick. These days, I blog here at Georneys about geology, and I blog over at Skepchick about other skeptical and scientific things, such as the elusive tree octopus. Occasionally, I will re-post some of my old geology posts from Skepchick. I will also sometimes cross-post on both blogs. The post below originally appeared on Skepchick in 2009 here. I thought I would re-post this because, if you haven't figured it out already, peridotite is my favorite rock and peridot is my favorite gemstone. If you want to learn more about where peridotite (and some peridot gems) come from, be sure to read last week's geology word O is for Ophiolite. After this, I will try not to write about peridotite and peridot for awhile-- though no promises since they are my favorite rock and gemstone.

Peridot gemstone. Image taken from here.

def. Peridot:
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, peridot has a beautiful green color which I love.

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.