Amethyst or Alexandrite What is the difference?

Answer 1

Amethyst is a variety of the colorless mineral quartz whose chemical formula is SiO2 (silicon dioxide). Its purple hue comes from minuscule amounts of impurities. It is a 7 on the hardness scale.

Alexandrite is a variety of the mineral chrysoberyl whose chemical formula is BeAl2O4 (an oxide of beryllium and aluminum). It is considered a green to yellowish green mineral in natural daylight, but its color appears dark pink to purple/cherry in artificial incandescent light. This property comes from minuscule impurities of Cr(III) (chromium in the +3 oxidation state) that reflect the excess green wavelengths found in the spectrum of natural sunlight and reflect the excess red found in the spectrum of incandescent light. Ordinary chrysoberyl does not demonstrate this so-called alexandrite effect. It is an 8.5 on the hardness scale.

The two mineral species also occupy different crystal systems, broadly defined by the symmetry of their atomic arrangements. Quartz, and thus amethyst, belongs to the trigonal system, and chrysoberyl, and thus alexandrite, belongs to the orthorhombic system.

Quartz is one of the most common minerals on earth (think of sand), though amethyst is far more rare, and chrysoberyl is also a relative rarity. Natural alexandrite on the other hand, comes from less than a couple of dozen localities in the world, and this scarcity drives its high price. Artificial or synthetic alexandrites are marketed commonly because of the high price of the natural ones. The only similarities shared by amethyst and alexandrite are that they are birthstones and that they are valuable because they are durable, scarce, and attractive.