Birth Stone

Month / Stone ---15th- 20th century

January garnet
February amethyst, hyacinth, pearl
March heliotrope, jasper
April diamond, sapphire
May emerald, agate
June cat's eye, turquoise, agate
July turquoise, onyx
August sardonyx, carnelian, moonstone, topaz
September chrysolite
October opal, aquamarine
November topaz, pearl
December heliotrope, ruby

Month / Stone ---U.S. (1912)
January garnet
February amethyst
March heliotrope, aquamarine
April diamond
May emerald
June pearl, moonstone
July ruby
August sardonyx, peridot
September sapphire
October opal, tourmaline
November topaz
December turquoise, lapis lazuli

Month / Stone --- U.S. (2013)
January garnet
February amethyst
March aquamarine, heliotrope
April diamond
May emerald
June pearl, moonstone, alexandrite
July ruby
August peridot
September sapphire
October opal, tourmaline
November topaz, citrine
December turquoise, zircon, tanzanite

Month / Stone --- Britain (2013)
January garnet
February amethyst
March aquamarine, heliotrope
April diamond, rock crystal
May emerald, chrysoprase
June pearl, moonstone
July ruby, carnelian
August peridot, sardonyx
September sapphire, lapis lazuli
October opal
November topaz, citrine
December tanzanite, turquoise

Month / Stone --- Hindu
January serpent stone
February chandrakanta
March Gold Siva-linga
April diamond
May emerald
June pearl
July sapphire
August ruby
September zircon
October coral
November cat's-eye
December topaz

Zodiacal
Early civilizations, such as those in India and Babylon, have attributed gemstones with magical properties and especially to their relationship with the signs of the zodiac. Over time, astrologers assigned gems of certain colors to the twelve signs of the zodiac to help people influence the planets in their favor.

Tropical zodiac
Sign / Dates /Stone
Aquarius 21 January – 21 February garnet
Pisces 22 February – 21 March amethyst
Aries 22 March – 20 April heliotrope
Taurus 21 April – 21 May sapphire
Gemini 22 May – 21 June agate
Cancer 21 June – 22 July emerald
Leo 23 July – 22 August onyx
Virgo 23 August – 22 September carnelian
Libra 23 September – 23 October chrysolite
Scorpio 24 October – 21 November beryl
Sagittarius 22 November – 21 December topaz
Capricorn 22 December – 21 January ruby

Birthday (day of the week) stones
While this word has also been used as synonym of Birth stone (see above), there is a separate list of assignment according to the day of the week of the recipient's birth:

Monday: pearl, crystal
Tuesday: ruby, emerald
Wednesday: amethyst, lodestone
Thursday: sapphire, carnelian
Friday: emerald, cat's eye
Saturday: turquoise, diamond
Sunday: topaz, diamond