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.

A Peridot is Forever

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 why I am thankful for science. 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 2006 here. Since I recently blogged about my favorite rock peridotite, I think it is timely to revisit this Skepchick post. The Skepchick post appears below with minor corrections.

A peridot engagement ring.

As a geologist and a skepchick, I’d like to debunk a popular myth: a diamond is not forever. This clever phrase was coined in 1948 as a marketing slogan for DeBeers, but I cry that it’s false advertising. Diamonds, formed at high-pressures deep within the Earth, are metastable in the low pressures at Earth’s surface. Over many million of years, the carbon atoms in the gemstones will rearrange, converting the diamonds into graphite. Pencil lead, essentially.

A real forever gemstone? Maybe zircon. There are zircons in places such as western Australia that are over four billion years old. Forever is a long time, though, and even zircons eventually degrade. Blue zircons can make really beautiful gemstones, though.