Modified: April 2011
by Jerry Sisk, GG; Co-Founder, Jewelry Television®
If you are thinking a YAG may be one of those big furry creatures seen on the Mongolian tundra during National Graphic specials, then you may want to read a little farther. YAG is actually a man-made gemstone that has been around for quite a few years. The name is a little strange but, truthfully, that is not unusual in the world of gemstones.
To start with, YAG (rhymes with lag) is not referred to by the individual letters. YAG is an acronym for three words--yttrium (pronounced it-tree-um), aluminum, and garnet. As you may have noticed, that’s quite a mouthful, so it didn’t take too long for someone to come up with a shorter term. The name is somewhat misleading since YAG is not really a garnet. True garnets are silicates. All six species in the garnet group contain an SiO4 molecule in their chemical formula.
Let’s start with a little information on the scientific side. YAG’s structure is similar to that of garnet, but its chemical formula is somewhat less complex. It is made up of three elements, two of which are found in the acronym (yttrium and aluminum). The final member of the trio is oxygen. The chemical formula is represented as Y3Al5O12.
While the elements aluminum and oxygen are fairly well known, the rare-earth element yttrium is not. As exotic and futuristic as its name may sound, yttrium was discovered in 1794 and has had a long history with the scientific community. It appears on the periodic table of elements, abbreviated with the symbol Y. It is also a vital component of the chemical formula.
YAG is synthesized under controlled conditions using a melt process. In simple terms, all the ingredients are heated in a crucible until the solids liquefy. A rod is then inserted, pulled slowly upward and rotated. As the rod is retracted, the material cools. A large crystal, called a boule, is formed in this way.
YAG is different from many synthesized materials because it has no natural counterpart. It is therefore considered an imitation gemstone, a material that is used to simulate the appearance of other more expensive creations of nature.
Created in the 1950s, YAG did not make its way into the jewelry trade until many years later. Its first application was in the fields of optics and laser technology, but many new uses have been found in recent years. When YAG finally made its debut in the jewelry trade, it was marketed as a man-made diamond simulant.
In its original form, YAG was colorless, or nearly so, and reasonably hard, ranking an 8 on the Mohs scale. One of the first things jewelers noticed was the heft. YAG is nearly 30 percent denser than diamond (SG 4.57 to 3.51). It has a reasonably high refractive index, but the dispersion is .028 to .044 for diamond. That means YAG does not exhibit the level of fire that is so characteristic of diamond. It is attractive, but not quite as lively as the king of gemstones. It was, however, the best diamond simulant at that time.
Shortly after YAG’s introduction to the world of jewelry, cubic zirconia came on the scene and replaced it as the simulant of choice for diamond. CZ is slightly harder and has higher dispersion, but YAG still lives on.
Then manufacturers started experimenting with various dopants, which are elements added in trace amounts to the “standard ingredients.” They are used to alter certain characteristics of the material being synthesized. In the case of YAG, dopants were added to alter color. By varying the recipe, manufacturers could produce pastel to vibrant hues ranging from various shades of yellow to yellow green, green to blue, pink, and even black.
One of the most popular colors, green, has been marketed as a simulant for the green tsavorite and demantoid garnets. YAG can be a beautiful yet inexpensive alternative to the more expensive gem-quality garnets. Blue YAG, which is also highly popular, can rival the appearance of fine aqua.