The Egyptians have been using turquoise as ornamentation for thousands of years.  It has been so widely simulated by the most diverse materials, and even the ancient Egyptians imitated it with a ceramic material ...So, as a buyer, how do you "know" that you are purchasing the "real thing?"  The only way to identify turquoise with absolute certainty, is to take it to an expert.   However, as a serious collector of gemstones and jewelry, I have learned a few things over the years that have helped me with my purchases, and I hope they will help you make qualified decisions on purchasing turquoise.

Appearance

On the rare occasion when it has not been interfered with in any way, turquoise has a uniform surface appearace, almost like that of unglazed china or very fine grained, homogeneous rock.  It may be a strong blue color, but is more often pale sky blue, (called robin's egg,) greenish blue or pale green. 

It would be very wise to invest in a loupe (magnifying glass,) as the cost in nominal, around $10.00, but it will become invaluable in determining everything from a stones texture and veining, to whether the prongs are loose on your diamond ring.  I have both a 10x one, and another 4x power one that I took the above photo through.  The stone on the concho is 12 x 9 mm, and only a fraction of the metal was photographed.  You should also purchase a meter and inches guage, which will give you accurate sizes on your stones.

Below is an 12 x 8 mm earring has a robin's egg blue colored stone with very fine grain and no veining.  I paid  about $50.00 for the pair back in the late seventies. 

Be aware that dyes and artificial resins may be soaked into the stones to improve color and hardness, and they do tend to reduce the destinct coldness associated with real gemstones.   If you have been handling it, allow the stone to lay on a surface for a few minutes, then press it against your cheek,  it should feel very cool or cold for about five to ten seconds.  Any less time, it could have a lot of wax fillers, or it is plastic.  Note the settings that the stones are in, as above, a fine sterling silver or 14k gold setting is a pretty good indication that it is real. 

The asking price is another sign about the quality of the stone(s) or piece.  The problem you will have on ebay is that the prices sometimes start out low, and you can't hold or feel the stone for cold, so you have to rely on the seller's feedback score, and remember to always make your purchase only using Paypal. 

It is not turquoise if it appears under a lens to consist of numerous, minute grains of polygonal shape, juxtaposed in an artificial manner, with a homogeneous blue or heterogeneous light and dark blue or light blue and whitish color; or if it reacts in a matter of seconds, ten at most, to a drop of hydrocholoric acid (don't do this, much too dangerous for an untrained person), showing fairly strong effervescence, a change in color, or obvious surface damage. (all this must be verified under a lens.)  I don't own a lens like this, only a qualified gemologist or jeweler would be able to make this determination.

It is not turquoise if it is warm to the touch, light like plastic, and burns with the characteristic odor of plastic when touched by a thin piece of red-hot iron wire, or if it has obvious seams where the plastic was molded.  Look for tell tale fine light scratches, usually with dirt in them.  Real turquoise will have large chunks of black matrix, clearly marked lines, cracks and creavases.

Turquoise can contain narrow veins of other matrix materials, either isolated or as a network, (called spider webbing,) these are usually black or brown, though sometimes yellowish brown. 

The stones are often used in the rough form (see above), polished, with a simple hole drilled through and strung together for necklaces, the smaller stones pictured above are in the green blue to pale green range, and are usually should be inexpensive to purchase.   The larger rough stones are of course heavier, and as are generally strung into necklaces which can get pricey.  The below example was strung with rough coral nuggets all in graduated sizes. 

The stones are a very pale blue with darker blue veining and cobwebs on some.  I purchased several necklaces over the years and combined them with the purchased coral to make an astonishing piece.

Turquoise is also cut and spherically or summarily rounded, polished  and made into cabochons, carved gems, or figurines.   The cabochan below has the fine brown lines of  the matrix veins, and various shades of color from pale blue to darker were purchased in pairs.  Even in the large 16 x 12mm they are light weight enough to wear comfortably as earrings.  Earring backings are available to allow you to make your own jewelry.

When, as often happens, the turquoise has been impregnated with paraffin, the surface appearance under a lens is distinctive, with small, whitish, opaque patches juxtaposed with and interpenetrated by bluer, translucent areas, sometimes they appear set against a faint pattern of larger, indented patches. Like all gems which are basically pastel in color, the richer-colored types are the most appreciated. 

The preferred color is a strong sky blue, the pale greenish blue being less highly sought after, and the pale green even less, used to be the rule, however, the beauty of the stone is in the eye of the beholder, and it is your vision that counts.

  This true Native American made Squash Blossom necklace (turquoise stones color is actually much deeper and bluer than photo, are all the ones set in silver, other colored beads are made of blown glass,) it cost me $400.00 back in the 1970's.  I recently restrung it with the glass beads to get a longer look and to be able to wear it more often. 

Apple green Turquoise has recently come to my attention, and I think it is absolutely gorgeous.  The ring I purchased has a huge 25 x 20 mm stone, and is set in fine sterling silver. It has fine light brown veining (see below,) and since I have worn and chipped it, I can see that it is uniform in color, and that it is not dyed howlite.  I believe the stone was stabilized and possibly the color was enhanced, but I love the look, and I am satisfied with the purchase.

It really comes down to buying what you like, and trusting the ebay and paypal system to work.  Use common sense and buy only what you can afford and when you get it home, if you test and find it is not genuine, as the seller described, you have recourse though a refund from the seller and/or you can get assistance to recover your money from Paypal. 

Credit to Simon & Schuster's guide to Gems and Precious Stones, by Kennie Lyman; and ACN Special Edition, Gemstones of the World, revised and expanded edition by Walter Schumann.