The new “lead-glass composite rubies” are still being sold as genuine treated rubies, so here we will set the record straight. They are very different from “treated rubies” and not understanding how they differ can be costly to every jeweler or designer.
The following is provided to help eliminate the confusion about these products—whatever they are being called—and help provide some clarity on how they differ from ruby or “treated” ruby. Here we will refer to them as “composite” ruby.
The presence of lead-glass makes them much more vulnerable to damage than treated rubies, because lead-glass is softer than other types of glass, and more easily damaged by chemicals. The lead-glass also has a specially formulated refractive index (RI) that is almost identical to ruby; this means that no one can see where one begins and the other ends. This is also why the treatment is much more effective in creating the appearance of fine ruby, and why a gemologist testing the stone’s identify with a refractometer will see the RI for “ruby” and thus identify it as ruby unless other tests are also used.
1. What is “composite” ruby?
“Composite” rubies are created by using chemicals on very low-quality corundum to leach out extraneous minerals and debris. This step is followed by infusing molten lead-glass into the spaces and voids created by the removal of the unwanted material from the corundum.
2. How do these compare to the value of natural or treated?
These are much cheaper than any “natural” or “treated” ruby, even rubies treated by extreme methods. Initially they could be purchased at $1-$4 per carat; three years ago, most could be purchased for $2–$6 per carat in sizes up to three carats; two years ago, prices were $4–$8 per carat; today prices are 3–4 times the original price, some exceeding $50 per carat, reflecting the dramatic rise in demand by buyers who still do not understand what they are actually buying, and vendors are charging what the market will “bear”!
3. How much lead-glass is in a “composite” ruby?
The percentage of lead-glass typically ranges from about 15% to over 50%, but any stone that contains any amount of lead-glass must be identified as a “composite” ruby, even in cases where the amount of lead-glass appears to be minimal. This is because the refractive index (R.I.) of the lead-glass matches ruby. This means that as light moves through the stone, you cannot see where one substance ends and the other begins. For this reason, in lead-glass products, one can’t even precisely determine just how much glass is in the stone. Even X-rays, which are used by labs to detect the presence of lead, fail to provide precise information pertaining to the volume of glass in a specific stone because the lead is simply one component of the glass.
4. Does the presence of lead-glass affect color and clarity grading?
Since the lead glass is tinted and cannot be removed from the stone, it is not possible to accurately and reliably grade the color. In “treated” ruby, the purpose of using glass fillers is to reduce the visibility of fractures and colorless glass is typically used; this will not affect grading the color. In cases where one might be concerned about the use of a tinted filler, the glass can be removed to determine the true color and then the stone can be re-filled. This is not the case with lead-glass composites; lead-glass cannot be removed at all without destroying or severely and irreparably, damaging the stone.
With regard to clarity, the very high R.I. of the lead-glass – which as we stated above is formulated to match the R.I. of corundum – makes it impossible to grade the stone’s clarity because having matching RIs makes it impossible to actually see where the fractures are and how serious they might be (for more information about a stone’s RI, see below). One can’t determine how deep or wide—how dangerous—any fracture(s) or void(s) might be. Even a single fracture can be extremely dangerous and make the stone much more fragile and vulnerable to breaking, depending on where it is located and how far it penetrates into the stone. And if there is just “one” fracture that contains lead-glass (which, by the way, we’ve never seen) it is vulnerable to progressive acid-etching at the jeweler’s work bench – which cannot be reversed or repaired, and will seriously mar the stone’s beauty. In the case of “treated” ruby, since the glass used has a lower RI than ruby, it is easy to see so it poses no problem in terms of clarity grading.
The reason these lead-glass products appear so clean and bright is precisely because the R.I. of lead-glass is almost a perfect match to that of ruby!
What is RI?
The R.I. of a stone relates to how the light moves through, and between, different media—in the case of these composites, how it moves through ruby and glass. The greater the difference between the R.I. of each substance, the more easily you can see the different components; the closer the R.I., the more difficult it is to see them. If the R.I. is essentially the same for both substances, you cannot distinguish where one ends and the other begins. This is why other types of glass sometimes seen in ruby and sapphire (usually silica glass) are different; they have lower R.I.s so you can actually see where any fracture is and properly grade the stone’s clarity.
5. Does the presence of lead-glass affect the weight indicated for “composite” rubies? How do they compare to “treated” rubies that contain glass?
The “represented weight” for composite rubies is totally misrepresented in all cases. The weight indicated does not represent the weight of the ruby but, rather, the combined weight of the ruby plus however much glass is present. To make matters worse, lead-glass weighs approximately 1.5 times more than ruby or sapphire. So the actual weight of the ruby or sapphire in any of these new composites will always be less than the indicated weight and depending on the percentage of lead- glass in the stone, the actual weight might be significantly less than what is indicated.
By comparison, “treated” rubies typically contain silica glass, and only miniscule amounts of glass are present, either as residue in surface reaching fissures as a result of very high heat techniques requiring that the stone be coated with borax for protection, (which melts and forms the glass residues) or in fractures that have been filled with glass to reduce their visibility or reflectivity (called “fracture-filled” or “in-filled” ruby).
Traditionally “treated” rubies containing glass have much less glass than lead-glass composites, which can be seen with a loupe/microscope so it is easy to determine how little glass is present. As a result, the glass present in traditionally treated rubies has very little, if any, impact on a stone’s weight.
6. Are lead-glass ruby composites more fragile than “natural” or “treated” rubies?
Yes. Coming into contact with a variety of common household substances and various types of surfaces that pose no threat to “natural ruby,” or rubies treated by routine techniques, have proved disastrous for composite rubies.
Substances such as lemon juice and other solutions commonly found in households or on a jeweler’s bench can quickly, easily, and irreparably damage these stones. Even lemon juice that accidentally gets splattered onto one of these stones while squeezing a lemon into iced tea or onto a salad or fish can etch the glass if not removed before removing and storing the jewelry, creating an ugly and undesirable stone that cannot be repaired.
Lead-glass is softer than other types of glass, and much softer than corundum, so these “composite” rubies are much more vulnerable to chipping and scratching from contact with any harder surface. Lead-glass hides cracks from view so they are more prone to being broken if accidentally knocked where the fracture reaches the surface of the stone. Facet edges are more easily abraded, becoming dull and unattractive.
Treated-rubies typically are not easily damaged because they are essentially “ruby” and retain the physical and chemical properties of ruby.
7. Is damage to a “composite” ruby permanent and irreparable?
Yes. Damage to composites is permanent and irreparable. This is not the case with natural and traditionally treated rubies; jewelry repairs or settings are much less likely to be damaged in the normal course of wear, but in unusual cases where damage does occur, it is normally easy to repair with minimal loss of weight or value.
8. How high is the lead content in the glass used in these composites?
Below you can see how the quantity of lead present in the glass itself affects the R.I.—the more lead, the higher the R.I. This also shows that the percentage of lead present in the glass formulation used to create theses new “composite” rubies (and now sapphire as well) is very high; in order to create a glass with an RI that matches that of corundum, which has an RI of 1.76–1.77, the amount of lead used in the formulation must be around 60+ percent.
R.I.’s for Various Glasses
Glass, Fused Silica – RI 1.459
Glass, Pyrex – RI 1.474
Glass, Flint, 29% lead – RI 1.569
Glass, Flint, 55% lead – RI 1.669
Glass, Flint, 71% lead – RI 1.805
The good news is that they are easy to spot and separate from natural or treated ruby. I will cover how to spot them in my next article. JBA
Antoinette Matlins is a professional Gemologist (PG) and Fellow of the Gemmological Association of Great Britain (FGA) with over 25 years experience with diamonds, colored gemstones, pearls, and fine jewelry. Member of the Board of the Accredited Gemologists Association (AGA). Author of 7 books on gems and jewelry, and a leading consumer advocate in the field. This article is taken from Gem Identification Made Easy, 5th Edition: A Hands-On Guide to More Confident Buying & Selling © 2013 by Antoinette L. Matlins, PG, FGA & A.C. Bonnano, FGA, ASA, MGA. Permission granted by GemStone Press, www.gemstonepress.com.