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All the birthstones are minerals, except pearls.

Minerals are created in the earth. Pearls are considered "organic" because they form inside a living organisms, various species of underwater mollusks. Another example of an organic gem is amber. Amber began life as tree sap. After millions of years, it turned into the gem material we know today.

Typically, lapidaries cut bloodstone, lapis, opal, sardonyx, and turquoise as cabochons. Nevertheless, all birthstones could receive cabochon cuts (except pearls, which are never cut). However, gem cutters more commonly facet the transparent birthstones (alexandrite, amethyst, aquamarine, citrine, diamond, emerald, garnet, ruby, sapphire, tanzanite, topaz, tourmaline, and zircon).

Cutting en cabochon, or "cutting cabs," is the most common form of gem cutting. Cabs have a flat bottom and a curved or domed top. Gem cutters usually cut cabs from opaque gems or transparent ones with many inclusions, or materials trapped inside the stones.

If you can envision a typical diamond engagement ring, you're probably imagining a faceted gem. Gem cutters typically facet transparent gems. That means they cut the gem so that many flat, geometrically arranged surfaces cover it. Each of these flat surfaces is called a facet.

Faceting will bring out a gem's brilliance. This term refers to a gem reflecting the light entering it back to the viewer. Don't confuse brilliance with "fire" or dispersion. This term refers to the multicolored flashes you see coming out of diamonds and some other gems.

Typically, diamond, emerald, ruby and sapphire command the highest prices. However, the question of value has no simple answer.

The Four Cs

A gem's value depends on four qualities, commonly known as the Four Cs.

• Color. Usually, the richer the color, the higher the value. However, with diamonds, the closer they come to colorless, the higher they are in value.
• Clarity. If completely transparent, a gem will be worth more than a hazy specimen. Virtually all gems have inclusions, things inside them, such as fractures or bits of other minerals. Some are so small, they can't be seen and have little or no effect on the value. Others are big and ugly or can weaken the stone.
• Carat. This is the gem's weight. Usually, big gems are rarer than little ones.
• Cut. The quality of a gem's cut has a great effect on beauty and value.

Although many consumers consider diamonds the most rare and expensive gem, that's not truly the case. Rubies that weigh over a carat are rarer than diamonds of that size. A 3-ct ruby, with excellent color and clarity, can fetch more than a diamond of the same size and clarity.

Emeralds grow fairly large. Thus, size isn't as important a consideration as with ruby. However, emerald gems almost always have visible inclusions. A top colored diamond, with the same amount of inclusions as a top colored emerald, would have less value.

Faceting Affects Value

Gem faceting quality greatly affects beauty and value. Just imagine a diamond that sparkles with fire and light. Next to it, imagine a diamond that looks dull, like a piece of cut glass. That's the difference faceting can make.