In antiquity, precious stones were diamond, ruby, emerald, sapphire and pearl, sometimes the opal was also considered a precious stone. Everything else is considered semi-precious gemstones. Among the popular semi-precious stones and less expensive ones are the amethyst, turquoise, aquamarine, topaz, peridot, opal, tourmaline, alexandrite, garnet (but the green garnet could cost more.) Today we still use these terms in the world of jewelry.

So our initial understanding is that precious stones are of greater quality, of greater importance and higher priced, and semi-precious meaning of less commercial value than precious. This clearly indicates that these terms are marketing terms, and ultimately confusing. Many semi-precious stones such as the rare and beautiful Tanzanite, may cost as much as $1000 per carat. So don't be deceived as to think that because it is called semi-precious that it will be less expensive. A diamond can be purchased for as little as $200 per carat while a garnet which has always been regarded as semi-precious could be selling over $500 per carat. Therefore the labels precious and semi-precious should be completely abandoned because whether a gemstone is considered precious or semi-precious has very little to do with the current value. These labels put on gemstones are inaccurate and can often be misleading. As the example of the Tanzanite, many gemstones today could easily be categorized as precious due to their increasing scarcity, and high price. Rarity will be an important factor in determining the cost of a gemstone and the value of a gemstone should not increase or diminish just because it is categorized as precious or semi-precious.

Precious and semi-precious are nothing but labels helping us to categorize gemstones, but in no way determine importance or value. As far as terms go, here are some you shoul know: "gem", "gemstone", "jewel" "synthetic" and "simulant".

Gem: a mineral (naturally occurring material) cut and polished.
Gemstone: a fragment of the gem (the cut stone).
Jewel: the combination of metal and gemstone. Gemstone set in a metal in order to be worn.
Synthetic: a man made material in no way equivalent to a mineral.
Simulant: a material resembling another material, example glass used as a substitute for ruby or emerald.