I am not a native English speaker, (Dutch living in Spain, hence
karat instead of carat) I checked google but still have no idea
what the stone was pigue means!
Pique (q, not g) is an old fashioned (prior to the GIA diamond
grading system coming into widespread use) term for "included",
meaning the stone as internal "feathers", or cracks, included
crystals of diamond or other minerals, or other internal defects.
Usually the term means stones with fairly significant inclusions, so
they are visible to the naked eye.
I assume that you mean the crack was already there when she bought
Could be, and likely was, or could be damage due to accident of some
type. Sizing the ring is not likely to have caused the damage, with
the exception of stones that have been treated to enhance their
apparent clarity. That's done by filling those cracks that reach the
surface, with a glass material that closely matches the optics of
diamond, making the cracks very much less visible. That type of
filling can concievably be damaged in sizing or other repair, if the
goldsmith gets the stone hot, as in "soldering temperatures". This is
not common in ring sizing, but it's not impossible either, especially
if the goldsmith was not as highly skilled as he/she should have
Maybe so, but is there a solution for this problem? In Europe or
what can I advise her? Can I still clean the ring without risk?
If the crack is very severe, you're safer not cleaning it, but
instead referring her to some other jeweler with perhaps greater
experience with diamonds of this type. Just because the potential
liabilities of a stone this size should be considered. But in
general, most diamonds can be safely cleaned. The material is quite
tough, and withstands normal handling well.
Repairing the crack to prevent further cracking is not significantly
possible. It may be possible to have the crack filled, as noted
above, but this does not add strength or stability to the stone. It
just makes the crack less visible.
What is the best method in this case?
Soak it in a mild cleaning solution (hot) for a bit, rinse it clean
(avoid the ultrasonic cleaner, and if you use a steam cleaner, do so
at a slight distance so the stone is not blasted with high
temperature steam), and give it back to her, advising her to seek a
jeweler who has formal gemological training in order to more fully
evaluate the stone's condition and prospects.