The quality of the gem also determines whether it is considered precious or semi-precious. There are gemstones, such as garnets and tourmalines, that are more beautiful, rare, durable, and costly than the 'precious' gems. A small, fine quality deep green garnet called tsavorite would be more costly than a low-quality emerald; a brilliant red spinel more costly than a low or medium quality ruby. And a rare, brilliant, “neon” tourmaline from Paraiba, Brazil, will cost as much or more than a rare Burma sapphire.

There is no precise defintion of what a 'gemstone' is. It is defined largely by the rarity and value of the stone. Some well known semi-precious gemstones are:

Remember your 4C's when you buy gemstones - COLOR, CARAT, CLARITY, and CUTTING.

Color is all important in colored gems. The "best color" is complex, being a combination of hue, saturation and tone. Don't assume that the darker the color, the better the stone. That isn't true: color can be too dark, like some rubys that look more brownish or orangish red than blood red. The more bright and vivid the color, the better. In precise grading terms: clear, medium-tone, intense and saturated primary colors are the most preferred. Muted colors and colors between hues, which you might find very attractive, are usually less expensive. Look at the color in different kinds of light.

Clarity is the color and 'clearness' of the gem. Hold it up to the light. Can you see lots of black specks (inclusions). Clear transparent gemstones with no visible flaws are the most valued. There is no standardized grading system for clarity: it varies by gem variety. With colored gemstones, if the inclusion doesn't show in the face up position, it generally doesn't matter at all. (unlike diamonds which are graded upside-down at 10x magnification). Some varieties, notably emerald and red tourmaline, are very rare without inclusions of some kind so the price structure takes this into account. Pastel colored gemstones show inclusions more, so they generally detract more from the value for pale stones. In rare cases, inclusions can increase value. Special effects like the star in star sapphire and the eye in cat's-eye chrysoberyl are caused by inclusions. Inclusions can also be a birthmark, proving that a gemstone is from a particular place. So "horse-tail" inclusions in demantoid garnet make it more valuable because they prove it came from Russia.

Carat is the weight of the stone. Gemstones are sold by weight, not by size. Prices are calculated per carat. A carat is one-fifth of a gram. Some gems are denser than others so the same weight stone may be a different size! For example a one-carat emerald is a bigger than a one-carat ruby. Just like diamonds, the carat weight also affects the price: large gemstones are more rare, so the price per carat is higher. But practically, this doesn't make much of a difference with common gems like amethyst, citrine and blue topaz. It really kicks in for ruby, emerald, sapphire, alexandrite, tsavorite and demantoid garnet, Paraiba and rubellite tourmaline, spinel, and pink topaz. Anything above 100 carats is classed as a collector's stone. Far too big to be mounted in a necklace or ring, it's a stone that you keep wrapped up in cotton wool, and get out on special occasions.

Finally, the cutting. Are all the facets of the stone equal and well-cut, giving the gem life and sparkle. A gemstone with a good cut is something that may not cost more but can have a positive or negative effect on it's beauty. A well-cut faceted gemstone reflects the light back evenly across its surface area when held face up. If the stone is too deep and narrow, areas will be dark (aka extinction). If it is too shallow and wide, parts of the stone will be washed out and lifeless (aka window). The best way to judge cut is to look at similar gemstones next to each other. Dark areas in the stone are caused by light leaking out the back of the stones because the angles aren't right. Look for a stone having even brilliance. But the cut affects the pattern of light you will see.