Turquoise is a completely opaque blue gem, revered throughout Asia and the Arab countries both for its great beauty and its alleged therapeutic properties. In the Arab cultures it was believed to protect the wearer from the evil eye, contagious diseases and poison. Amulets studded with turquoise were plastered to the door-frame to steer evil influences away from the house. In Asia , according to the Buddhist beliefs, the gem was considered to be blessed by Buddha and a promoter of strength, courage and love. The best quality turquoise in found in the northeastern region of Iran , in the region near Nishapur. This is also the oldest source of the gem, dating back atleast 3000 years that we know of. Poorer grades of turquoise are found in Mexico , the U.S, the Sinai Peninsula in Egypt and China . It is an extremely dense gem and distinctive for the evenness of its color. These are the main criteria for judging the quality of the stone- the purity of its blue color, the dense, heavy texture it should have and the evenness of color distribution across the surface. These great features sadly encourage the mass- production of imitations, usually made of glass. It is not very difficult to create imitation turquoise and anyone purchasing this stone
Turquoise ranges in color from very light cerulean blue to dark cobalt blue and green-blue. The most prized gems have intensely blue color with pale green undertones and the poor grade gems have shallow color or are sullied with yellow undertones. This gem is also susceptible to line-inclusions termed ‘spider-webbing' that mar the perfect blur hue. Thin white lines interweave across the surface in a web-like formation, weakening the stone. This flaw is debatable, as there is as much of a market for spider-lined turquoise as there is for solidly
Another inherit problem with the gem is it's stability. This is a fairly soft stone, rating only 6 on the Moh scale of hardness and it's inherently vulnerable for this reason. Turquoise found in Iran is considered the best not only because of its color, but because it doesn't display this problem. Other countries produce weak, porous stone that crumbles easily or loses color within a year of being mined. It may even change color completely, turning white, pale green or even brown. Furthermore, the stone tends to age; turning either darker with time or strengthening its green undertones. This is one of the reasons that turquoise from any part of the world besides Iran is treated for color. Today the U.S produces fine-quality turquoise that has good color and is reasonably stable, but it still doesn't rival the Iran-produced stone. Also, many fraudulent stones circulate the market, made up of powdered turquoise or unstable stone material bonded with glue or plastic, coated for durability and treated for stability. This is very common for this gem, as are synthetic reproductions.