Turquoise is a vibrant light-blue-to-green-gemstone with a hardness of 7 on the Mohs scale. It is often fracture sealed, which means it is sealed with acrylic resin to enhance its color, texture, and hardness, not that it has fractures.
Turquoise made in a laboratory has a very uniform color, and chalky varieties of turquoise are often coated with wax or oil to enhance its color. When thus treated, the color change is not permanent.
Under heat (roughly about 500 degrees Celsius), blue turquoise will adopt a greenish hue. Turquoise is porous and will absorb oils (including skin oils) over time, gradually changing to a yellowish color.
Turquoise supposedly prevents procrastination, and it has been thought to warn its wearer of danger or illness by changing color. Turquoise is the anniversary gemstone for the 5th year of marriage.
Turquoise is somewhat soft, so avoid scratches and sharp blows. To be certain that no coatings are removed, also avoid hot water and household chemicals.