Geuda is the milky corundum, formerly discarded as worthless by the Sinhalese cutting industry that constitutes the greatest reserve of high quality heat-treatable material available. Experts believe Sri Lanka, by common consenses, has the world’s largest deposits of this particular type of corundum rough.
The term geuda itself refers to a semi-transparent to semi-translucent corundum which appears silky or milky under reflected light (normal viewing conditions), but viewed under transmitted light (the light traveling through the stone before reaching the eye) gives a characteristic ‘tea’ color known in the trade as the ‘diesel’. Both these effects are due to the presence of impurities within the crystal, that under the correct heat treatment procedures it can (if the concentrations are correct) give rise to a strong blue coloration and an increase in transparency.
Milky Or Silky Effect
This is due to the presence of inclusions of rutile (TiO²). These may be in the form of long slender needles or as clouds of minute particles.
Diesel Or Tea Effect
Iron oxide impurities give rise to brownish patches or streaks that may or may not be randomly distributed through the crystal. Sometimes the oxide patches will be oriented parallel to the crystal faces.
Together these two sets of impurties have the potential to cause blue coloration if unlocked from the structure of the crystal and allowed to combine. These color causing agents (iron and titanium) may be activated by heating the corundum under certain conditions at around 1650ºC. If the most favorable ratio of iron/titanium is present a fine blue color of uniform distribution may be achieved, as will considerable improvements in transparency and luster. In general, the degree of milkiness and the intensity of the diesel effect are proportional to the color that results from heat treatment. Less diesel will result in a pale or lighter shade of blue, while intense diesel will give a dark blue color. If there is too much silk or milkiness, it is possible that some will remain after heating, which will influence the eventual transparency.
Local Classification Of Geuda
Over the years a considerable number of names, and a local classification have evolved. Here are a number of the most frequently used terms:
Diesel Geuda: Milky white intense brownish diesel.
Silky Geuda: Intense inclusions of rutile in the form of silk. May often display a strong diesel effect. Often further classified by its body color (yellow silky geuda)
Waxy Geuda: Material with waxy or dull appearance. May show a moderate diesel effect.
Milky Geuda: Dull white milky appearance with a diesel effect. The term blue geuda will refer to a blue milkiness with a diesel effect, while yellow geuda will indicate material with a yellowish milkiness, and so on.
Young Geuda: May be any body color, with a small amount of silk and/or diesel effect.
Thick Geuda: Opaque material with characteristic intense milkiness or diesel effect causing a dramatic reduction in transparency.
Dalan Geuda: The lowest grade of material made of mixed varieties (rejects) of geuda, usually with little silk or diesel effect. The lowest potential for successful treatment.
These coloquial names are based on the degree of concentration of the milkiness (observed under reflected light) and the intensity of the diesel effect (observed with transmitted light). The terms can be quite subjective, however, with different dealers categorizing the same materials under different names. For instance a milky geuda with a intense diesel effect may be described as diesel or milky.
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