This fine blue stone, although opaque, has been rated by some as a precious stone but despite its undoubted attraction and its rarity iri fine pieces, it cannot really compare with the transparent blue stones, such as the blue sapphire. Its color, a deep sky blue at its best, is one that is ever popular, for a fine blue is not com­mon in gem stones.

The general run of turquoises is, however, inclined to be a greenish blue to an apple green in color, and naturally these are not so attractive as the deeper blues. Veins and marks, general intrusions of the parent rock, often spoil pieces which otherwise would be good, although some prefer such markings if they are not too prominent since they give character to the stone. The natural color of turquoise, which is probably due to its copper constituent remains constant in artificial light, a property which its many imitations do not possess.

As a jewel stone, however, turquoise possesses many disadvan­tages. It is porous, and it is therefore readily affected by liquids. Perspiration, alone, will gradually change many of the blue stones to a dull, unattractive green. Nearly all fade in course of time and in wear, and heat is particularly detrimental. These effects may be due to the chemical composition of the stone, which is very complex but which always contains a proportion of water. Chemically, it is a complex hydrous phosphate of aluminum and copper, with ferric oxide replacing some of the alumina.

Not only do most stones become paler in color when constantly exposed to sunlight but many pieces fade immediately they are mined. The original color may be temporarily restored by burying these stones in damp earth, or by staining through immersion in a solution of ammonia. Berlin blue is also sometimes used, but such methods never have permanent effects.