Thursday, 30 May 2013

Yttrium Aluminium Garnet

4.8 cm bar of colorless YAG. Aluminate of yttrium and aluminium with a garnet type structure it is an artificial product with no natural counterpart

Crystal system

Cubic

Appearance 

It is produced by the Czochralsky process according to which crystals in the form of cylindrical rods about 4-5 centimeters in diameter and 20 centimeters long are "pulled" from the melt. It is perfectly transparent and colorless, but green, yellow and blue forms have also been produced by adding small quantities of other elements.

Physical properties 

It has a hardness of slightly more than 8. The density is about 4.55 9/cm3. The refractive index of 1.83 is quite high, but much less than that of diamond and cubic zirconia. The dispersion of 0.028 is tar lower than that of diamond.

Production  

It is mainly produced in the United States.

YAG (colorless)

This was the chief substitute for diamond in the 1960s. It has now been replaced by other products, mainly cubic zirconia.

Appearance 

Perfectly transparent and colorless and singly refractive like diamond, it is usually seen cut into brilliants; more rarely, it is given a step cut. Its luster is about the same as that of colorless zircon and greater than that of corundum and spinet. Being hard but not brittle, it is easily given a high polish and sharply defined edges. But low dispersion makes these stones look a bit lifeless, particularly those with a step cut.

Distinctive features

Round brilliant of colorless YAG, 2.10 ct.Because YAG's refractive index is quite a bit lower than that of diamond, if a brilliant cut stone is viewed from above and tilted gradually, a characteristic dark area will appear in the table facet (opposite the ob-server), corresponding to certain facets of the pavilion which let the light through instead of reflecting it. Further-more, if the stone is viewed from the table facet (with a lens or microscope) the distinct reflections of the table and crown facets will not be visible as they are with diamond. Also, if step cut stones are placed upside down with the table facet resting on a printed page, the letters will be visible through it. The girdle is not finished like that of diamond; it is not cylindrical and is usually striated, as if it had been filed; yet, despite this rough finish, it looks fairly transparent. A hardness test using a corundum-tipped pencil, which is midway between the two, will readily distinguish YAG from diamond.

Cost

One of the lowest, equal to that of the least expensive synthetics The cost of the raw material is, in fact, very low and it is much quicker and easier to cut than diamond, being softer and without any tendency to brittleness.

Writer – Curzio Cipriani & Alessandro Borelli

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