January - Garnet (Constancy)
Said to ward off depression and treat arthritis, anaemia, and when green, for sinus ailments eye sight and soreness of the breast. The Garnet was extremely popular with the Victorians and is the main stone related to Aquarius, but Amber, Amazonite and Hematite can be found to provide healing functions for those born in this month too, the Capricorn and the Aquarian. If you are a January baby the Diamond will bring confidence in personal abilities in this month with the White Sapphire providing much needed support in work, and the Falcon's Eye providing that creative spark.
February - Amethyst & Aquamarine (Sincerity)
The Amethyst protects from accidents, excess and disease, and used to treat upper and middle back pain, diabetes and fatigue. Preserves affection. The Holy Grail was believed to be made from this stone whilst the Aquamarine promotes examination of the 'life-force' within the wearer, often used for the Egyptian Eye of Horus to promote inner vision. The Aquamarine is used to treat laryngitis, nerves, hypoglycaemia, menstruation, constipation, asthma and anorexia.
March - Bloodstone (Courage)
Used to treat many conditions and illnesses including lower back pain, anaemia, baldness, diabetes and blood pressure. The Diamond, the Ruby, the Emerald and the Amethyst bring healing functions for those born in this month in the house of Aries. The White Opal brings strength to visions and ideas, Jade calms fears, Pearl brings consolation and the Amethyst will bring protection from illness for Pisces in this month.
April - Diamond (Purity)
Symbolic of love and affection, strength in men and pride in women. Believed to ensure longevity and protection against mental illness in ancient times. Once known as the Philosopher's Stone by alchemists. This is the stone most closely associated with the house of Aries. The Diamond brings confidence, the Emerald aids communication and the Amethyst will alleviate headaches in this month for the Aries.
May - Emerald (Hope)
Once believed to ensure fertility, symbolic of resurrection, indicates inner vision. Egyptian Eye of Horus often portrayed with this stone. In ancient Egypt the Emerald symbolised resurrection and reincarnation. Reputed to protect against eye infections, stress and diseases of the blood. Once used like the Turquoise to promote passion and arousal. The Emerald is the stone most closely associated with the house of Taurus. Here the Rose Quartz and Lapis Lazuli bring further positive qualities of imagination inspired and strengthens action. The house of Gemini lays claim on the end of the month and is associated with the Citrine, Amber, and Tourmaline.
June - Pearl & Agate (Health)
Pearl engagement rings can bring misfortune due to the changing energies of the moon upon the person, but embodies feminine innocence, purity and sweetness. Agate reputed to ward of negative energies, and once believed to provide invisibility. The Agate has close associations with the house of Gemini and at this time the Citrine, Amber and the Tourmaline aid Communication, imagination and concentration.
July - Ruby & Moonstone (Contentment)
Changes of colour in a Ruby said to indicate the health of the wearer, and channel energy. The stone has also been used to treat headaches. Moonstone was believed to cure lunacy, yet could also bring mixed fortune due to its associations with the changing forces of the moon upon the wearer. The Moonstone and the Pearl have close associations with the house of Cancer which also brings the Peridot. Associations with the Yellow Topaz, Tiger's Eye, Ruby, and Milky Yellow Amber are important for those born in the house of Leo in this month. Relief from inner stress, received vitality, strengthening of the heart and a firm base of thought are provided by these stones.
August - Sardonyx or Peridot (Fidelity)
Said to provide courage and protection against attack, and drive out sadness. The name of the stone translates as 'conjugal felicity'. The Ruby has close associations with the house of Leo in this month. Virgo brings associations of Amethyst, Carnelian, Pyrite to this month. Freedom from worry, stress, and the strengthening of energies are provided here by these stones.
September - Sapphire (Repentance)
Considered to be a stone which brings wisdom and strengthens intuition encouraging depth of thought. Also seen as a protection against negative forces. Has been used to treat headaches. Sardonyx also has close associations with the month of September as it is the main stone for the house of Virgo as well as Leo. Pyrite is particularly important to Virgo at this time.
October - Opal (Loveable)
Believed to be able to encourage the 'third eye', often used by mystic's. Also known as the 'stone of hope and justice'. To be worn with Diamond if not born in this month. Seen as a stone to indicate health and bring good fortune in the East. The Opal will bring energy and clear decision making and Jade will strengthen the inner self and the ability to understand the impact of one's actions with care and compassion.
November - Topaz & Malachite (Cheerfulness)
Topaz brings good fortune symbolising the sun in financial terms and in relationships. Said to promote courage and wisdom. Malachite provides the ability to understand the speech of animals, and promotes spiritual well being in times of depression. The Topaz and Malachite have close associations with the house of Scorpio. The Ruby will bring inner strength this month, and courage, whilst the Garnet will exert balance on passionate whims in the material, spiritual and physical realms. The Carnelian will make sure you keep your feet on the ground at this high point in your year, whilst the Black Pearl will calm anxieties.
December - Turquoise (Unselfishness)
Brings happiness, and prosperous relationships between friends when given as a gift, and can also indicate changes in the health of the wearer. Once used to promote love and arousal! The Diamond, the Falcon's Eye and the White Sapphire can provide healing functions for those born in the house of Capricorn. For Sagittarius, the Turquoise brings protection against any form of problem this month, whilst the Amethyst will develop understanding of self and others.
This interpretation of gemstone meaning was found here, but there are many different interpretations of the meaning of gemstones. This site has an exhaustive resource of all the gems. Study for yourself or throw it all out and go for something you just like.