January: Garnet
Red Garnet is possibly the most affordable amongst gems. Dark Red is the color that most commonly comes to mind when garnet is mentions, but this gem can be found in a variety of colors including bright orange and green.
Garnets are found in just about every color except blue.

February: Amethyst
Relatively common, Amethyst offers affordable beauty. Found in a variety of purple shades, the deep royal purples have been admired by kings and queens for centuries.

March: Aquamarine
The darker the blue the more valuable, Aquamarine is most often found in a pastel blue shade. More affordable versions of this gem include a green tinge. Cuts you will commonly find are oval and emerald.

April: Diamond
The Greek word "Adamas " means unconquerable and indestructible. This is the origin of the word Diamond. It is one of the most indestructible natural formations known to man. From this word and from the depths of the earth is derived The Diamond.

May: Emerald
Rare and valuable, Emerald is most often cut into a rectangular step-cut, commonly known as the emerald cut.

June: Alexandrite
This gem features a color change that is considered the most dramatic amongst gems. The best examples of the alexandrite appear green in daylight or fluorescent light changing to a red or purplish red in incandescent light from a candle flame or lamp.

July: Ruby
Rubies are very rare and valuable in large sizes. A fine four carat ruby will generally sell for more than a top four carat emerald or diamond.

August: Peridot
Found in the rocks created by volcanoes and even in meteorites that fall to the earth, Peridot is the extreme gem instantly recognizable by it’s “lime-green” color.

September: Sapphire
Best known to be found in every shade of blue, sapphires also come naturally in every other color except red. Associated with romance and royalty, the engagement ring given to Lady Diana Spencer from Prince Charles was Sapphire.

October: Pink Tourmaline
Pink is of the most common in the tourmaline family, making it a more affordable gem. Tourmaline is durable and suitable for everyday wear.

October: Opal
Offering a rainbow of hues including those of every other gemstone combined, Opals are featured in the crown jewels of France.

November: Topaz
Topaz is also available in a wide variety of colors. Because blue topaz is so readily available and affordable it can be found in more unusual cuts. 
The most valuable topaz is more red than yellow, from a pale pink to a “sherry” red.

December: Blue Zircon
Although colorless when chemically pure, zircon is typically yellow-brown to brown. It also comes in red, blue, purple, and green. Because of its high brilliance and dispersion, colorless zircon has sometimes been used as a substitute for diamond in jewelry.

**Source: GIA/www.gia.edu