Birthstones

January: Garnet -- Red, darkish-brown, violet-red, pale yellow-orange, orange, orange-red, transparent orange-brown, translucent green, and bright green (most valued). Though the mineral garnet is common, garnets that possess the qualities required of a gem are rare.

February: Amethyst -- Mostly purple, can sometimes be a bluish-violet color

Amethyst is usually found in pointed crystals containing six sides. Iron and manganese are believed to be the reasoning behind the purple color. 

March: Aquamarine -- Mostly Blue

Aquamarine is a transparent gem that is a bluish-green color. The name aquamarine came from the Latin word meaning "sea".

April: Diamond -- Usually transparent in color, sometimes can be blue

Diamonds are the hardest of the gemstones, as it is the hardest natural mineral.  Diamonds consist of almost pure carbon and because of this, will not dissolve in acid.  It can however, be destroyed if placed in extreme heat. 

May: Emerald – Green in color

Possessing a big, perfect emerald can hold almost as much value as possessing a small diamond.  Emeralds have 6 sides, and typically come from Colombia. 

June: Pearl -- Usually white in color

Nacre (NAY kur) is what makes a pearl. If an oyster or mollusk that produces nacre comes into contact with a particle, the oyster or mollusk will cover the particle with nacre.  The particle is them trapped covered with the nacre forming a sphere and essentially a pearl.  

July: Ruby -- Usually Red in color

Rubies are the rarest gemstones.  Rubies are usually given as wedding anniversary gifts, as it is the symbol for the fortieth wedding anniversary.

August: Peridot -- Mostly green in color

The mineral olivine usually produces this gemstone.  Peridot is one of the oldest gemstones as it has known and valued since biblical times.

September: Sapphire -- Any Color, but mostly blue. A large sapphire will be almost as valuable as a small diamond.  Star sapphires are the most valuable sapphires, and are named star sapphires because of the way they reflect light almost like stars. 

October: Opal -- Any, Mostly white

New South Wales, Australia produces the most valuable opals, which are the glowing black opals. If held for an extended period of time in dry air, an opal will crack. 

November: Topaz -- From yellow in color to light brown to pinkish red color

Topaz is another transparent gemstone and is a hard gem as well.  Most topazes will be white or blue in color. Topaz is often sold in very large stones, and has been known to come in thousands of carats.  Topaz is a much harder and more luminous gemstone than quartz.

December: Turquoise -- Bluish-green, sometimes changes in color

Turquoise is a brightly colored gemstone that is very popular among the Navajo designed jewelry.  Turquoise is of high value in the Navajo culture because the gem is believed to bring the people closer to their Gods and also to protect them from harm.  In ancient Aztec culture, they showed their rank by wearing turquoise. 
Turquoise comes in a variety of colors, though turquoise that is sky-blue in color is the most valuable.