Judging Quality of Rubies and Sapphires (RWH)
Next
to red diamonds, rubies are probably the
most
expensive of gems in the world
in sizes
over 3 ct.
The ideal for ruby is that of a red traffic
signal,
a highly fluorescent red of high intensity.
Unlike
diamond, small amounts of silk
in a stone
actually
help the colour because they
scatter light into areas it would not
otherwise go.
This
helps cover up the extinc- tion
which would
otherwise be found.
7.01 carat Mogok ruby |
Thai/Cambodian rubies suffer the double
deficiency of too much iron,which cuts the
fluorescence, and no silk to scatter light.
Thai / Cambodian ruby
Top prices for blue sapphire are paid for stones
of an intense blue verg- ing on the violet.
Top color from the left |
Large sapphires are more common than large
rubies.
Colour
zoning can be a problem in
sapphire: look for stones which have no major
zoning problems. Many blue sapphires are too
dark in colour, especially
those from basalt
sources such as Thailand and Australia.
Such stones are generally inexpensive.
color zoned sapphire |
Similarly, sapphires which are too light in colour
are also inexpensive.
The best stones are those which are well cut, eye
clean and, most importantly, with intense blue
colours.
Natural Sapphire with outstanding color and clarity |
Orange sapphires should be rich in colour
without blackish areas.
In star stones, transparency is an important
factor.
Sri Lanka Star Sapphire |
Too much silk results in a lowering of colour
intensity, because the length of light paths
through the stone is reduced
due to scattering
from the silk.
The ideal star stone contains
just enough silk
to show a star, but not enough
to cause the colour’s intensity to
be reduced.
A stone can be expensive
if its colour is good,
even if the star
is poor. In contrast, stones with
sharp stars but poor colour are not valuable.
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