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Treated Ruby. Heat treatment The most common treatment for natural rubies is heat treatment. We consider any ruby that we deal with to be heat-treated unless we can prove otherwise. This treatment is done to improve the colour or clarity of the stone. Heat changes the colour of a gem by altering the chemical state of some of the stone's atoms. In the case of rubies, colour can be uneven--too brownish--or the crystal may have a blue core. (Many Mong Hsu rubies have blue cores.) Temperatures vary depending upon the material being treated and the desired result, but can go as high as 1,800°C/3,272°F. Heat treatment may also improve the clarity of a ruby by dissolving inclusions (usually rutile) back into the crystal. Sometimes the opposite is done: A stone is heated at lower temperatures (1,100°C-1,500°C/2,012°F-2,732°F) in order to cause the rutile to crystallize, resulting in a star stone. Identifying heat treatment in ruby requires the ability to recognize a stone that has not been heat-treated. Untreated rubies have clusters of fine rutile needles or silk in their structure that are easily visible using a microscope. When heated, these needles tend to break or form dotted lines within the stone (figure 4). This "broken silk" is the easiest way to spot heat treatment in a ruby. (Be aware, though, that not all rubies have silk.) Other indicators are pitted girdles and double girdles. Pitting often occurs on the surface of a stone as a result of the high heat used in treatment. Generally, this pitting is polished out, but some may remain on the girdle of the stone. Double girdles result from this repolishing process, when a slightly different angle is used on the girdle than was used originally in cutting the stone. Double girdles are an indication, not proof, of heat treatment, as there could be other explanations for their presence. Exploded inclusions are another result of heat treatment. An exploded inclusion is caused when crystals or liquid within the stone expand and contract at different rates than the surrounding material, causing a fracture. Keep in mind that not every inclusion explodes during heat treatment: the likelihood varies depending on the temperature of treatment and the particular identity of an inclusion. As a result, certain types of intact inclusions indicate lack of treatment, while others may not. In general, inclusions surrounded by fractures are strong evidence of heat treatment. Caution is needed in making this identification, though, since naturally occurring fractures can be mistaken for stress fractures caused by treatment. The best way to practice identification is to observe as many heat-treated stones as possible. Other treatments Some ruby treatments less common than heat treatment
are diffusion treatment (see below under sapphire), filling of fractures
with red oil, or glass filling of cavities. The first two treatments
are relatively uncommon, but glass filling of cavities is common and
is somewhat controversial. Ruby is often packed in borax during heat
treatment. The borax melts, forming a glass-like substance that fills
surface reaching fractures. There has been debate as to whether this
filling is purposeful or a byproduct of the heat treatment process.
Either way, the lower polish of the glass-filled cavities is readily
seen with a microscope when using reflected light on the ruby surface.
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