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.