A gemstone is a piece of mineral, which, in cut and polished form, is used to make jewelry. However certain rocks (such as lapis lazuli), or organic materials that are not minerals (such as amber), are also used for jewelry. Rarity is an important characteristic that lends value to a gemstone. Apart from jewelry, from earliest antiquity until the 19th century engraved gems and hardstone carvings such as cups were major luxury art forms.
An article with definitions for all gems can be found here. Wikipedia article on gemstones is also a good read.
HOROSCOPES AND GEMSTONES
|Capricorn||(Dec 22 - Jan 20)||Garnet, Onyx, Ruby, Turquoise, Blue Topaz, Lapis Lazuli|
|Aquarius||(Jan 21 - Feb 19)||Amethyst, Garnet, Jasper|
|Pisces||(Feb 20 - Mar 20)||Aquamarine, Amethyst, Ruby, Bloodstone, Jasper|
|Aries||(Mar 21 - Apr 20)||Diamond, Bloodstone, Topaz Sapphire, Jasper, and Aquamarine|
|Taurus||(Apr 21 - May 20)||Emerald, Sapphire, Garnet Agate|
|Gemini||(May 21 - June 21)||Pearl, Emerald Alexandrite, Agate, Ruby|
|Cancer||(June 22 - July 22)||Ruby, Emerald, Ruby Sardonyx, Pearl, Moonstone|
|Leo||(July 23 - Aug 23)||Peridot, Onyx, Diamond, Carnelian, Sardonyx, Ruby|
|Virgo||(Aug 24 - Sept 23)||Sapphire, Carnelian, Zircon, Sardonyx, Peridot, Agate|
|Libra||(Sept 24 - Oct 23)||Opal, Peridot, Agate Aquamarine, Tourmaline, Sapphire|
|Scorpio||(Oct 24 - Nov 22)||Topaz, Beryl, Opal, Amethyst, Aquamarine, Tourmaline|
|Sagittarius||(Nov 23 - Dec 21)||Turquoise, Topaz, Beryl, Blue Topaz, Blue Zircon, Ruby, Lapis Lazuli, Citrine|
Early civilizations in both India and Babylon as far back as 3000 BC attributed rare and beautiful gemstones with magical properties. The early gem buyers sought advice from astrologers first, in hopes that they could influence the forces that were identified with the planets in their favor.