Gemstones can be described generally as those crystalline minerals valued mostly for their beauty and their use in jewelry. Most have industrial uses, and most can be synthesized as well as mined. Synthetic gems are not considered as valuable as natural gems, but they work just as well in industry.
The two kinds of gemstones are precious and semiprecious. These are historical classifications not necessarily indicating commercial value. A rare kind of semiprecious gem may fetch a higher price than a low-grade precious gem.
Gemstones can be graded according to the 4 Cs: color, clarity, carat, and cut.
Color is a property of the gem material. White light is a mixture of colors of the spectrum, as we can see in the rainbow. When the material absorbs some colors, we see the remaining colors reflected or transmitted. Pure, intense colors are valued the most. Complete absence of color, such as in a clear diamond, is also valuable.
Clarity is how transparent the gem is. The precious gems are potentially very transparent, but flaws will make any gem appear dimmer or more cloudy.
Carat is the weight of the gem. One carat is 200 milligrams or about 141.7 to the ounce. Size may make the gem more valuable, but it is not a primary concern.
Cut is the shape and design of the gem. Gems are cut into various shapes and given particular reflective facets for two reasons. The first reason is to enhance the beauty of the gem. Opaque gems are usually ground and polished into smooth rounded shapes, and translucent gems are cut with facets. Certain angles of facets are best for reflecting light that falls on the gem back to the eye of the viewer. The second reason is the purpose of the gem, whether it is to go into a setting, such as a ring, or whether it will be surrounded by other gems. The original shape of the stone and any internal flaws will also be important considerations.
Precious gems are the most valuable because of their beauty and rarity. There are only a few:
Sapphires are equivalent to rubies with a different color, usually blue, and are discussed there.
Emeralds are a green precious version of the stone beryl.
The types of semi-precious gems are numerous. Just some are listed here:
Amethyst, a purple variety of quartz
Beryl, a crystal containing beryllium
Garnet, a group of common to rare gems
Jade, an opaque green stone
Topaz, a clear or tinted hard gem
Birthstones are a bit of folklore with no deep cultural significance. Your birthstone is associated with your birth month, and it is simply a suggested gift. The list of birthstones has varied from place to place and over time, but here is one set: