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YAG
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 It has now been replaced by other products, mainly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 spinel. 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 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 observer),
corresponding to certain facets of the pavilion which let
the light through instead of reflecting it. Furthermore, 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 (9), 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.
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