When buying turquoise jewelry, it is good to know a few of the basics about this most popular gemstone, from how it is identified, named, and graded; how the mined rock can be altered; what imitation stones exist; and how to avoid overpaying.
Turquoise is often named according to where it is mined, but that’s not always the case. Bisbee Turquoise is so named because turquoise was found during open-pit copper-mining operations in Bisbee, Arizona. But turquoise can also be named after a person, a tribe, a province, or a country (Persian turquoise, Chinese turquoise, Mexican turquoise), in which case, you won’t know exactly where the turquoise was found outside of knowing it is from that country.
According to Joe Dan and Joe P. Lowry, authors of the excellent reference book Turquoise: The World Story of a Fascinating Gem, Persian turquoise is the most mentioned and the most famous turquoise in the world. Right behind Persian Blue, they say, is Cerrillos Turquoise, which comes from the Cerrillos Hills near Santa Fe, New Mexico. Also desirable is Number Eight Turquoise from the Number Eight Mine near Carlin, Nevada. Number Eight Turquoise is recognized by its clearly delineated spider web matrix. Why the name “Number Eight”? The story goes that the eighth claim was the best one.
So you see that naming can sometimes be a capricious thing, and will not always give you the best information regarding the value of any particular turquoise. What makes a piece of turquoise valuable is its grading system, which is based on:
Condition: natural turquoise is valuable; any turquoise that’s been treated is less valuable.
Source: turquoise from the “Classic Mines” -- i.e. Bisbee, Lander Blue, Lone Mountain, Number Eight, Morenci -- are worth more.
Color: darker colors are more rare; blue is generally more sought after; but green has a market too within its own mines.
Clarity: no visible host rock is most valuable.
Matrix: symmetrical and aesthetically placed on the finished cabochon is most valuable.
Size: A cabochon that is larger is worth more.
I want to mention density and hardness too, because that comes up later in the discussion about stabilization. To quote the Dan and Joe Lowry:
“Natural high-grade turquoise generally has a density that protects it from the infiltration of foreign substances and should never change color” (172).
This is SO important to know, because a lot of turquoise marketing will tell you otherwise.
What are the ways that turquoise is altered?
Turning rough turquoise into cabochons for jewelry takes a skilled lapidary who can use many techniques to alter the stone. There are several ways lapidary processes modify turquoise. These processes are not necessarily all bad, but a good consumer of turquoise should be aware of them.
Bonding – if a cut cab has holes or pits, they can be filled in with a mixture of crushed rock and bonding agents. Turquoise that has been bonded should be disclosed to buyers. Ask about it.
Dyeing the matrix – dark matrix is more desirable than white or light brown matrix, so it is dyed black or dark brown. Turquoise that has dyed matrix should be disclosed to buyers. Ask about it.
Oiling/Wax Polishing – Medium- to low-quality turquoise cabs can be processed with oil or wax-based polishes, which means the lower-quality stone is improved, but will still absorb dirt and oils (p.175). With practice, you will be able to identify oiled or waxed turquoise. It will look waxy and not have shine. When you see it, know that it is not high quality and not natural.
Stabilizing – A process for medium- to low-grade turquoise, stabilization gets a bit complicated, but generally speaking it is a process that uses polymer resins via a variety of methods to penetrate the lower-quality turquoise so that it becomes hard and therefore resistant to breakage, dirt, and oils. There are many things to know about stabilization but here are three important things:
(1) Chalk stabilization– Dan and Joe Lowry say that fully “85 percent of total mining production of turquoise yields turquoise that is too soft to shape or carve and will not accept a high polish to its surface” (191). This type of turquoise is pale or white in color and is referred to as “chalk turquoise.” A portion of stabilized turquoise is chalk turquoise that has been dyed and then infused with resins/plastics.
(2) Clear shot stabilization- clear resins with no color dyes are used on medium- to low-grade turquoise. This is marketed as “natural turquoise,” but this is NOT TRUE, so don’t fall for the marketing that says you are buying "natural turquoise that has been stabilized.". Also, clear shot stabilization DOES change the color of the turquoise.
(3) Color shot stabilization – dyes are included in the resins to give the medium- to low-grade turquoise better color, using the same processes.
So here’s the problem with stabilization: A seller could disclose that the turquoise is stabilized, but might at the same time spin it as a “process to perfect natural turquoise,” which is categorically not true. While it’s independently true that making lower-quality turquoise harder and better able to withstand dirt and use can be a good thing, it is not a process used for high-grade turquoise, and certainly not a process used for natural turquoise, because as I pointed out with the Lowry quote, natural turquoise is already dense enough to withstand the “infiltration of foreign substances.” This is especially true if dyes were used to color-enhance the turquoise.
Enhancing – Enhanced turquoise usually refers to the “Zachary treat” process, which the Dan and Joe Lowry describe as a “process that takes medium-grade turquoise and treats it into a high-grade color-looking turquoise” (199). James E. Zachary’s processes of enhancement are proprietary, which presents a big identification problem, because it’s unclear to me that you can tell real, mined, high-grade turquoise from “Zachary enhanced” turquoise. Zachary will go so far as to describe his process as “micro-crystalline structurally enhanced” (201), for what its worth. In Turquoise: The World Story of a Fascinating Gem, the Zachary Treat and other enhancement processes are presented as an art form, and as such the results of the turquoise enhancement are themselves considered valuable. I’m thinking these enhanced items are not so much about jewelry as other art objects like tabletops and bowls and things, which are gorgeous.
Compressing – pieces and crumbs of turquoise are smashed together into a block that is stabilized and also possibly dyed. Lapidaries then create cabs from these blocks. Cabs made from compressed rock are easy to identify. Once you have seen one, you will know.
What are the common turquoise imitations?
Dyed Howlite (see photo above)
Howlite is a plentiful, porous gemstone that will take dye easily. Because it has something like a turquoise matrix already present in the stone, when dyed it can look very turquoise-like. Once you have been around a lot of turquoise, you can identify dyed howlite easily, because for one thing the matrix just doesn’t quite make it, and occasionally the dye job is less than impressive.
Dyed magnesite (see photo above)
Magnesite is used in a number of products, not the least of which is as a turquoise substitute. Similar to Howlite, it has a vein-like structure that mimics that of turquoise, and when dyed a robin’s egg blue, it can look a bit like turquoise. But once again, the result is underwhelming once you have been around real turquoise.
It is pretty easy to tell dyed howlite and dyed magnesite from real turquoise, but it can be hard to tell natural turquoise from altered turquoise, and that's where you could find yourself overpaying. For that, be aware of how lower-quality, altered turquoise is marketed as high-quality, natural turquoise. Establish yourself as a smart consumer by asking questions about dyeing (both the stone and the matrix), bonding, stabilizing, and the source of the turquoise.