The term semi-precious is used to refer to the entire world of colored gemstones, not including the four so-called precious gems, diamond, ruby, sapphire and emerald. But the terms precious and semi-precious are actually no longer used in the gem trade, and the American Gem Trade Association (AGTA) officially requires that "Members should avoid the use of the term 'semi-precious' in describing gemstones."
This rejection of the term semi-precious is a way of recognizing the incredible diversity and rarity of many colored gemstones. One of the major changes in today's gem market is a growing consumer awareness of the vast range of colored stones. These days customers ask us about sphene and iolite and other once-obscure gem varieties just as often as they ask us about sapphire or emerald.
So the world of colored gemstones needs some new classifications to replace the outdated distinction between precious and semi-precious gems. None of the trade organizations has proposed a new schema. But one is definitely needed, since we need a way to explain to buyers why some gems are highly valued (i.e., expensive) and others are not.
At the very least we need a classification for what we might call premium colored gems. These are gems which have excellent gemstone characteristics -- hardness, durability, brilliance, clarity and color. In most cases they are quite rare as well. You might think of these as the new precious gems. This group would include not only ruby, sapphire and emerald, but also spinel, tanzanite, tsavorite garnet, demantoid garnet, alexandrite and the finer tourmaline.
Another useful category is a group of gems that also have good gemstone characteristics, but are widely available and very affordable. These include members of the quartz group -- amethyst, citrine, ametrine, rose quartz, etc. -- as well as topaz and the more common garnets. All of these stones have very good hardness, between 7 and 8 on the Mohs scale, and most have very good clarity as well. But they are typically priced at well under $10 a carat.
That leaves 40 or more varieties of fascinating gems to explore. Gemologists of course have useful ways to categorize them, but the market has a hard time to grasp them under any concept except "colored stones." From andalusite to diopside to fluorite, and sphene to kyanite and moonstone, each has its own unique colors and gemological characteristics. As each variety becomes better known, connoisseurs seek out the finest examples of each variety, so that the very best peridot and rarest rhodolite themselves become premium gemstones.