Hone Your Ruby Radar

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Identifying glass-filled rubies at the bench

By Arthur Anton Skuratowicz

Ruby 5

Editor’s Note: How do you know when you have a fragile glass-filled ruby on your hands before you perform a routine repair that could destroy it? In this article, Arthur Anton Skuratowicz describes the 10 most common traits of these troublesome rubies and explains how to identify them at the bench. Since glass-filled rubies don’t respond well to common jewelry chemicals, polishing abrasives, and the heat from a torch, it’s essential to know when you are working with one of these stones.

 

Gas Bubbles Trapped in Large Glass-Filled Fractures and Cavities

With the exception of amber and obsidian (volcanic glass), natural gemstones do not have free-floating gas bubbles in the gem material. The glass filling used in these rubies will occasionally have gas bubbles trapped in it. This gives the appearance that the gas bubbles are in the gemstone itself. The gas bubbles look spherical and highly reflective. Because the interior of the bubble is reflective, it gives the appearance of having a dark center with a silver outline (think of the hole in a donut). The larger the bubbles, the larger the area that is filled.

Note: Synthetic flame fusion rubies may contain free-floating gas bubbles. These bubbles tend to be very small or in clouds, and the synthetic rubies grown in this fashion tend to look flawless. To verify if the ruby is synthetic, check for other synthetic characteristics, such as curved striae.

Ruby 1

It’s easy to see the bubbles in the glass filler of this ruby. The blue haze and wispy appearance are also visible.

 

Gas-Filled Voids Trapped in Fractures

Unlike gas bubbles, which tend to be spherical, gas-filled voids are larger and more random in shape. Caused by an incomplete filling of a fracture, gas-filled voids are globular or amoeba-like, but are still confined to the flat shape of the fracture. Depending on the angle from which they are viewed, they can look silvery or transparent.

Other natural inclusions may have similar looking voids, so it is best to look for other supporting characteristics when verifying if the ruby is glass-filled.

Ruby 2

The unfilled areas of the fracture have a different appearance than the rest of the ruby. Tilting the stone back and forth shows this best.

 

Blue Color Flash Along Fractures

Much like the filler used in fracture-filled diamonds, the glass filler material in ruby may show a characteristic blue color flash when viewed at different angles. It is important to view a filled fracture along its edge rather than through its flat surface. Viewing the fracture on its edge increases the probability of the blue flash occurring and recurring. An orangey yellow haze may be visible, but it is easily masked by the red color of the ruby.

Ruby 3

The subtle blue flashes are visible along the edges of these filled fractures. The bubbles support the glass filling identification.

 

A Blueish Haze Throughout the Ruby

Since many rubies that are glass-filled are also highly fractured, there is an abundance of blue flashes occurring throughout the stone and at multiple viewing angles, which may cause the stone to give off a blue haze.

Ruby 4

The blue flashes here make the stone look as if the center has a blue tinge to it. The bubbles support the glass filling identification.

 

Wispy Haze of Cobweb-like Inclusions Throughout the Ruby

The internal structure is very crackly due to the many fractures. Without the glass filler, the ruby would be barely transparent to nearly opaque. With glass filling, the fractures are made less visible but they are still there and affect the transparency of the stone. It looks like a wispy haze that has been described as cobwebs, shear curtains waving in the breeze, or cigarette smoke collecting in the air.

Use caution, as similar inclusions may be present in synthetic gemstones.

Ruby 5

The abundance of glass-filled fractures gives the appearance of veils or smoke.

 

Luster Difference on the Surface of the Ruby

Since the glass filler is not ruby, it is not as hard as ruby and therefore takes on a different polish and reflection. If a surface fracture or cavity is large enough, you may notice that it reflects back with a different level of luster. The luster of the glass filler is lower than the ruby and looks duller in reflected light.

Ruby 6

Reflecting light on the surface of this filled ruby shows the luster difference of the glass filler versus the ruby.

 

Glass Filler on the Surface of the Ruby

In some extreme examples, when little care was taken to fill the ruby adequately, there may be some residual glass filler on the surface. You won’t find this on faceted stones, but it may occasionally be present on carvings, crystals, or cabochons. It looks like transparent glue stuck to the surface.

Ruby 7

This glass-filled ruby crystal shows the glass material on the surface. It has the appearance of transparent glue or epoxy.

 

Previous Chemical, Heat, or Abrasive Damage

If the glass-filled ruby has been damaged previously, it is easy to see the extreme fracturing. If you see a multitude of cracks on the surface of the stone, do not proceed with standard bench techniques. This is a red flag--no matter what gemstone you are dealing with.

Ruby 8

This ruby was damaged after spending less than a minute in a pickle pot. The criss-cross frosty lines exhibit the high number of fractures reaching the surface.

 

Strange Gemological Property Behavior

The large amount of filler in the ruby may affect how the stone behaves gemologically. It is possible to use a refractometer on the stone, but the abundance of surface-reaching fractures may affect the clarity of the reading. A large cavity can have its own reading, which is different from that of ruby. In addition, using Polaroid filters, a Polariscope, or Dichroscope may prove difficult and offer confusing results.

Since the fractures and filler could mask or alter the double refraction or pleochroism of the ruby, take care when observing the ruby and use additional magnification to focus on a small area of the stone.

 

Price Is Too Good to be True

Lastly, many glass-filled rubies are sold at such "discounted" prices that it is difficult not to be suspicious. Compare prices and qualities with known and unknown vendors. Beware of embellished stories of new locations or lower manufacturing costs. If it looks like a thousand bucks but sells for $49.95, it’s too good to be true.