There's more to a gem than what's graded in the 4 "C's" (color, cut, clarity and carat weight). Many other factors should weigh in your decision to buy a stone. These are physical characteristics that make a good gem what it is: First, wearability or hardness. A stone should hold up to normal wear and not scratch easily. Our hands take such an everyday abuse, knocking around, grabbing, holding, scraping against everything from brick to steel, it's asking a lot for a stone to keep it's sparkle when it's mounted in a ring especially. Pendants usually don't go through such a daily beating, but some people still take off a nice pin or necklace and just toss it in a jewelry box or on a dresser to bang against other pieces, pocket change, keys or whatever else is there. So much depends on the wearer. I've sold rings that are back for repair in a six months, with the prongs beat up and stones chipped because the wearer didn't respect the beauty and character of the stone. Even with a good mounting that is heavy and protective, any stone can be damaged. Diamonds can be chipped. Even though diamond is the hardest stone, it is also brittle!
That brings up the second thing that makes a good stone, toughness. Toughness is a physical character that many stones exhibit even though they are not nearly as tough as a diamond. Brittleness and toughness and two different things entirely. A good example is aquamarine and peridot. Both are birthstones and two different species and different hardnesses. Aqua is quite tough and can be mounted and worn much easier than peridot which is more brittle. I could hammer a piece of jade with a considerable blow. The same force could be strong enough to break a diamond, because jade has a tougher character.
Heat sensitivity is another character that some stones exhibit -- opal is the worst problem child in this category. Any opal lover will tell you not to expose a fine opal ring to freezing cold and then immediately warm hands by a roaring fireplace! Many stones have some level of heat sensitivity, and while this is something that I must consider when cutting, normal everyday temperature variations are not very important with most species.
What stones make good jewelry? First, diamond is still king in hardness and it's generally tough enough that few are aware that it is somewhat brittle too. But why not enjoy some color in your life? The top stone after diamond has to be sapphire. Sapphire comes in ALL colors, not just blue! (Ruby is also included here, as ruby is just a red sapphire!) Sapphire is the next best thing in hardness, it's tough and has no heat sensitivity, cleavage or light problems. My absolute first choice in colored stones.
Second on my list is spinel. It's available in nearly all colors, it's hardness at 8 places it just below sapphire. It's tough, not heat sensitive, presents no problems and is VERY under rated.
My third pick is topaz. It's next in hardness at 8 but does present a problem with a perfect cleavage just like diamond. It is limited in color, but overall makes a very wearable stone.
Fourth is one of my favorite species: beryl. Gem beryl varieties are emerald (green), aquamarine(blue), heilodor(yellow), morganite(red), goshenite(clear) and bixbite(red) Beryl is 7 1/2 to 8 in hardness, it tough and not brittle or heat sensitive. The variations in color between the various available colors give a wide range to choose from.
The tourmaline group is another good choice because of the available colors, and hardness only slightly below beryl. However it is brittle and more heat sensitive.
Finally the quartz species rounds out the well know stones. Crystalline Varieties: amethyst, citrine, ametrine, rose, smokey, rutilated and the cryptocrystaline varieties: chalcedony, carnelian, sard, chrysoprase, agate, onyx, & jasper. Quartz has respectable hardness at 7; crystalline varieties are often brittle while cryptocrystaline forms are sometimes exceedingly tough.
This covers a good bit of the popular stones available, but is only the tip of the iceberg as there are many many more species not mentioned.

Carat is a measure the weight of gems. One carat is defined as 200 milligrams or 1/5 of a gram. Or another way of saying it is there are 5 carats in one gram. A carat is further divided into points. 100 points = 1 carat. So a stone that is said to be 50 points will weigh 1/2 ct. (.5 ct) or 100 mg. A stone that is 1/4 ct would be 25 points (.25 ct) or 50 mg.
Karat is a measure of the purity of gold. Gold is mixed or alloyed with other metals, such as copper, zinc, silver etc. to give it better wearing qualities and sometimes different color. Pure gold is 24 karat and is too soft to securely support and protect a fine gem. So to strengthen it gold is mixed with copper. When it is called 14 kt. it is 14/24 pure gold or 58.3% pure. 18 kt. gold is 18/24 pure or 75% pure.
Carrot is a measure of rabbit especially Bugs Bunny!

Plain gold rings can be buffed up with a small amount of red rouge (try lipstick!) on a soft cloth. For heavy scratches, it's best to stop by your favorite jeweler and ask if they will buff the ring for you. Most are happy to get you in the store and will not charge for this, but be sure to ask if you are not sure. They will also clean your stones at the same time or you can clean them yourself at home. For common faceted stones diamond, sapphire etc., use warm water and a grease cutting detergent such as Dawn. Use a small toothbrush and scrub the stone wherever you can reach it through the prongs. Increase the temperature of the water gradually until it is hot. In cleaning a stone, the trick is not to shock a stone by suddenly immersing it in extremely hot water. (Don't do this with opals!) Soaking in a detergent and ammonia solution may also help to loosen oil buildup on the stone. If you are serious about your jewelry, buy a small ultrasonic cleaner and use a solution of ammonia and a few drops of dawn in the water tank. A WORD OF CAUTION: I recommend NOT cleaning emeralds with an ultrasonic. Mined emeralds are usually treated and lower quality included/flawed stones may be damaged or actually fall apart in an ultrasonic! If you have a filled diamond it is also not recommended to clean with an ultrasonic! Most faceted stones should not be any problem however. Cabochons may generally be cleaned the same way. Start with warm water and gradually increase the temperature. Use a brush and some good detergent. Stones that are heat sensitive (opal), soft (amber) or porous (turquoise) should not be cleaned like this. When in doubt, it's best to go to the source if possible. The manufacturer, the one who cut the stone probably knows more about it than the salesman. If you can't find the cutter, I recommend the next person to ask is a GIA Gemologist. (Find one with the G.G.title if you can.) If you strike out on the first two, then be sure to ask your favorite independent jeweler. Last on the list would be chain and mall stores, because they often staff the sales counter with part timers and newbies with little experience and no practical educational background.

There are both federal and state laws concerning the stamping of jewelry quality. The National Stamping Act defines how a jewelry item is to be stamped for identification. However state laws may vary on the subject of quality and standards of fineness. The thing to remember is that articles are NOT required to carry a quality (karatage) stamp. When a manufacturer does not use a trademark identification, then the item of jewelry should NOT carry a quality stamp.

How is a stone graded for clarity?