Of all the natural substitutes, there are a few common culprits. To distinguish between these and natural turquoise, it is always good to know a bit of gemology.
Turquoise is often imitated by "fakes" such as the mineral chrysocolla. This is actually a part of the chalcedony group of stones, in which the mineral chrysocolla is finely disseminated, giving the chalcedony an intense medium blue green color. However, the refractive index (RI) of chalcedony is 1.53 to 1.54, much lower than that of turquoise at 1.61 to 1.65. You can test the RI with a refractometer to find out whether its chalcedony or turquoise. Whether its chrysocolla masquerading as turquoise or the other way around will depend on which is of higher value at the moment, you more liable to see turquoise dressed up as the more valuable chrysocolla
This also resembles turquoise and is light green to greenish blue in color with a distinctive mottling and grid like pattern not seen in turquoise. It also has a refractive index of 1.52 to 1.53, much lower than that of turquoise, and shows an uneven or splintery fracture as opposed to the conchoidal or granular fracture of turquoise.
It often has a veined or mottled appearance with a yellowish-brown matrix, a close enough resemblance to have earned the misnomers of Nevada or Californian turquoise. However, its refractive reading of 1.56 to 1.59 is much lower than that of turquoise and it will appear pinkish when viewed through a color filter, whereas turquoise shows no color change through a filter. Variscites color should be a deterrent in itself, as it resembles that of only poor-quality turquoise.
This is a naturally opaque white stone with a dark, spiderweb matrix appearance, similar to that of turquoise and therefore commonly dyed to imitate the more expensive stone. A quick and easy test to separate the two is a color filter; viewed through a filter, dyed howlite will appear pink or red. Howlite also has the lower refractive index of 1.58 to 1.60 to that of turquoise. As a third test, albeit a destructive one, a drop of hydrochloric acid on an obscure spot on dyed howlite will attack the stone and leave a distinct dull spot.
Synthetic Turquoise is virtually identical to natural turquoise but will show artificial-looking matrix as well as a distinctive darker blue, spotty appearance on a lighter background, visible fewer than 30 to 50x magnification. This appearance is better known as the 'cream of wheat' effect and is a dead give-away.
These will show a vitreous luster on small fractures as opposed to the waxy luster of the natural stone. Glass and plastic imitations will show signs of tiny bubbles under magnification. Touching plastic imitation turquoise with a hot point will melt the surface and emit an acrid odor. Another practice, one that can trace its origins back to 2000 B.C., is that of reconstituted turquoise. Known as faience during the Egyptian period, the material consisted of a quartz paste that was shaped, glazed, and fired to resemble sky-blue turquoise. Today, reconstituted turquoise consists of pulverized turquoise rock, mixed with resin and injected into a mold to form a solid brick, which is then shaped. Pulverized pyrite is sometimes added to better imitate natural turquoise.
Don't despair, though. The best solution is to buy your turquoise from a reputable dealer who won't hesitate to discuss treatments with you and supply you with a written guarantee that your purchase is nothing else but natural turquoise. Always purchase your turquoise jewelry from a reliable jeweler. Source: http://www.lapidaryjournal.com/feature/may03