About Gemstones

A Gemstone or Gem is normally a piece of crystal, but can also be a rock or an organic material which is cut and polished to make jewellery or other articles.
There are over 130 types of materials which have been used as Gemstones.
 
Gemstones are classified as Precious and Semi-Precious
In modern times, the precious stones are Diamond, Ruby, Sapphire and Emerald.
All other gemstones are considered semi-precious.
Amber and Jet are the major organic gemstones.
 
Typical well-known Gemstones are.
 

NameHardnessColourType
Agate 6.5-7.0 Various Colours Volcanic Rock
Alexandrite 8.5 Various Colours Oxide Minerals of Beryllium
Amber 2.5 Amber (Dense Yellow/Orange) Organic Resin
Aquamarine 7.5-8.0 Blue or Cyan Mineral (Beryllium)
Citrine 7.0 Yellow to Brown Mineral (Quartz)
Diamond 10.0 Yellow to Colourless Mineral (Carbon)
Emerald 7.5-8.0 Green Mineral (Beryl)
Garnet 6.5-7.5 All Colours Mineral (Silicate)
Jade 6.0-6.5 Green Mineral (Silicate)
Jet 2.5-4.0 Black Organic (Wood under Pressure)
Lapis 5.0-5.5 Blue Mottled with White Minerals (Mixture)
Moonstone 6.0 Pink, White, Yellow, Grey, Clear Minerals (Granite in Origin)
Onyx 7.0 Various Colours Volcanic Rock (Like Agate)
Opal 5.5-6.0 Various Colours Mineral (Silica)
Pearl 2.5-4.5 Various Colours Calcium Carbonate
Peridot 6.5-7.0 Yellow to Green Mineral (Silicate)
Quartz 7.0 Various Colours Mineral (Silicon)
Ruby 9.0 Red, Purple or Pink Mineral (Aluminium Oxide)
Sapphire 9.0 Blue Mineral (Aluminium Oxide)
SunStone 6.5-7.2 Various Colours Mineral (Silicate)
Tanzanite 6.5 Violet to Blue Mineral (Calcium Silicate)
Tiger's-Eye

5.5-6.0

Golden to Red-Brown Mineral (Silicon Dioxide)
Topaz 8.0 Various Colours Mineral (Silicate)
Turquoise 5.0-7.0 Blue to Green Mineral (Phosphate)
Zircon 7.5 Various Colours Mineral (Silicate)