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Turquoise
Mineral: copper aluminium phosphate
Formula: CuAI6(PO4)4(OH)8·4H2O  
Mohs: 5-6  
Colour: varies from opaque  ‘robin’s egg blue’, blue-green to green, with or without grey/black limonite  veining depending its source.  
Source: Mexico

Its blue is caused by the copper content and green with the amount of Iron included.  The un-veined variety is more popular in the near and middle East than the rest of the world. Its name is derived from the 16th century French for Turkey - Turqois or pierre turquin - dark-blue stone.  
Turquoise has long been prized as a gemstone in many ancient cultures from the Ancient Egyptions, Aztecs, Pe-Columbian Meso-Americans, Mesopotamian civilizations and China, through to modern day.  It is mentioned in both the Judeo and Christian Scripturs as on the ‘breast plate of judgment’ worn by Aaron representing one of the Tribes of Israil.    
As turquoise is a chalky and porous gemstone, it is normally treated with wax or oil to enhance its colour and durability and can, when heated, leave it covered with a whitish ‘bloom’. Native American Indian turquoise jewellery has a backing to increased the durability of thinly cut slabs and cabochons. As with most phosphate minerals, care should be taken when worn for the gemstone not to come into contact with chemicals of any nature, ie cosmetics, perfume, suntan creams and lotions, jewellery cleaning solutions, even natural skin secretions or prolonged exposure to sunlight, as they can cause damage discolouration and bleaching.
Be aware that there are other stones on the market that are sometimes sold as turquoise; such as dyed Prussion Blue Howlite or a reconstructed of powered turquoise bonded with a resin sometimes referred to as Turquinite.
Turquoise is mined in Iran, Southwest USA, Mexico, China, Chile, Austrailia, Afghanistan, Turkestan, Cornwall UK and Saxony Germany.  

Metaphysical
Turquoise known as a ‘Protection’ gemstone.
Astrological Sign: Sagittarius, Pisces & Scorpio
Keywords: wholeness, spiritual expansion, protection, communication  
Element: Storm  
Chakra:  Throat (5th)
Use: healer & protector of the Spirit. Provids soothing energy & bringing peace of mind.  Promotes self-forgiveness, self-acceptance & the release of useless regrets.  Helps to guild one through the unknown, protecting while promoting independence.   Imparts communication skill & protection against environmental pollutants.
Healing:  anorexia, headaches, throat, lungs, asthma, infection, teeth, hearing & high blood pressure, exhaustion, depression & panic attacks. Can be used as an elixir.
The information on the crystals & minerals has been researched from numerous sources.
If you would like to expand your knowledge further, there are several web sites and books we recommend:
Web Sites
Books
www.mindat.org
HEALING CRYSTALS
Michael Gienger Earthdancer Findhorn Press  ISBN 978-1-84409-067-9
www.wikipedia.org
LOVE IS IN THE EARTH  
Melody  Earth-Love publishing House  ISBN 0-9628190-3-4
www.dkwcrystals.co.uk
THE BOOK OF STONES
Robert Simmons & Naisha Ahsian  Heaven & Earth Publishing LLC   
ISBN 0-9621910-3-5
QUARTZ CRYSTALS
Isable Silveira Earthdancer Findhorn Press  ISBN 978-1-84409-148-5