Maybe this will be a bit helpful, from:
http://www.powwows.com/gathering/general-crafts/23371-how-can-you-tell-real-turquoise.html
(written by a forum user known as Mr. Bojangles)
Turquoise is a difficult stone to identify, and typically, ALL the stone out there is what is called "STABILIZED" turquoise.
Basically, what's done is, a, usually black type of plastic is injected into the stone. This enhances both the veining that is so highly sought after, plus, it helps to hold together an otherwise crappy piece of stone.
There are many mines throughout the world that produce good to crappy turquoise. Typically, where ever you have a copper mine, you'll turquoise being mined along side it as a side to the copper.
Copper is where turquoise gets some of it's color, I believe it's a type of oxidation that happens, giving you that blue to green coloration.
The more heavily veined, at least in Southwestern jewelry, supposedly, the more highly prized the stone is.
Bisbee and Morenci are old copper mines, both located in Southern Arizona, Morenci, according to many in the field of Southwestern jewelry.....and more importantly, the ones who actually manufacture it, the Navajos and other southwestern tribes. Value this stone, almost above all others. Cerillos is a mine located near Santa Fe, NM and is an ancient mine. Turquoise mining there dates back to pre-contact times, with alot of it being exported to Central America and the ancient centers there of the Aztecs, the Olmecs and the Toltecs....there is even some evidence of it having been among the Maya.
I know folks that buy up old jewelry and just rip the stones out and basically just melt down the silver or cash it in as scrap.
There is a thriving trade and business in just old stones. You pay by the gram, and typically, it's an upwards of $300 or more for a gram of Morenci. You have mines spreadout thru the West, in Nevada, California. Across the World, in Tibet, parts of the middle east and the Indo-Pakistani Subcontinent.
I know one test you could perform is a simple burn test. Heat a needle till it's cherry red and then stick the stone with it. If it penetrates.....you've got plastic.
Another test, is to take a file, run it across the stone, if it peels or mars...it's a low grade turquoise or a plastic.
Of course.....all this damages your stone, so, you might want to do it, you might not want to.
IF the stone is EXTREMELY BRIGHT BLUE, it's possible it's not turquoise.
Another thing you could do, if you know how, is remove the stone from the setting and look at the bottom. Stabilized turquoise, while infused with a black mastic, is still turquoise. It's done for just that purpose, to stabilize it, keep it from falling apart. Some coloration is also sometimes injected into a stone to enhance it before the mastic goes in. It's a fairly common, almost accepted practice....but, nothing is valued more than the PURE STONE, straight from the mine, worked or shaped raw into something.
Still, stabilized turquoise is turquoise.
If you should get the stone free from the setting, flip it over, typically, if it's a Southwestern piece, you'll probably find some sawdust under there, this is to create some resistance behind the stone so it pushes back against the setting or bezels, it also creates some "give" behind the stone so it allows for expansion and some "padding" behind the stone. Anyway...... if you have like a two millimeter layer of black plastic looking stuff behind the stone. It's turquoise, but it's the stabilized variety.
Another test I've heard of, but, I'm personally not going to endorse because it's so hard to use, and is inherently flawed. Is the finger test. Take a finger, put some rubbing alcohol on it to remove ALL the oil from your finger, dry it, and then rub the stones surface with your finger. Supposedly, a fake stone will have a waxy feel, like a piece of plastic. A real stone will have some resistance under your finger tip, it won't slide as easily.
I personally don't think it's a good test, but, I know several traders in Gallup that swear by it and buy and sell pieces according to this method. Over years and years of just looking at stones and after becoming familiar with veining and coloration as they appear in stones from various mines, a trader sometimes can distinguish what's real and what's not.
It's a tough business identifying real stones from enhanced or fake stones. Even traders get bilked into buying pieces that are made with fake or stabilized stones.