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.