So I was once told that the rating of gems as "precious" and "semi-precious" stones was something that dated back to the Ancient Greeks. After some research - as with most rumors in Art History - I no longer believe that to be the case.
It seems that the ratings came into existence in the late 1800s. Even if that is the case, however, I am still blown away that this rating still has the hold it has on the gem industry. Four stones - diamond, emerald, ruby, and sapphire are considered "precious" gems. All other gemstones are "semi-precious". It is that basic.
There are so many gems that are rare and at times more expensive than these - but just these four are at the top of their game (so to speak).
Throughout Art History there are the trends where pearls, lapis lazuli, carnelian, and other stones have all had their high ranking places in society. Now tastes and trends change with wind direction - but still the four gemstones stand strong in their rating.
Additionally, there is only one standard rating system in all of gemstones and that is for white diamonds. Nothing else that I have researched is standardized.
This has been especially helpful, for me personally, when I am shopping for gemstones all over the world. In fact, I recently purchased three types of "semi-precious" gemstones for a project - all round microfacted cut, all claimed to be AAA, and all were listed at the same size. I bought them from three different reputable vendors from three distinctly different parts of the world. And the beads I received are not even the same size, let alone the differences in quality and clarity.
So as with most things - buy what you like and buy what you think is pretty.