Pretty hard to tell, exactly, without a microscopic examination
of the break. But here are some possiblities:
Chlorine, as in swimming pools, bleach, etc, can attack white
golds. Would tend to find any microscopic flaws, and induce
brittleness wherever it could penatrate a bit. That can happen
anywhere. More of a problem with castings, but die struck heads
are not immune. Contamination with mercury, such as from a
broken thermometer, could do the same.
Is it possible this is a casting, made from a model using die
struck heads, and then just cleaned up really well? If so,
interior sections of the casting might tend to cool slower in the
casting process, which would induce larger crystal size. And the
last sections to solidify are sometimes starved for metal due to
shrinkage in the adjacent areas pulling metal out of any still
Over annealing, as in getting the metal too hot, too long, in
assembling the heads, can cause an increase in crystal size, and
brittleness. Also, white golds tend to be brittle when hot, so
if you solder together a piece and then quench it while it’s
still too hot, you can crack the metal. An interior section of
the ring might have remained a little hotter, a little longer,
in soldering the heads on, and if the ring were then quenched
while those prongs were still too hot, tiny cracks could have
started. However, this requires the gold to still be slightly
glowing red hot, or at least close to it, before it’s a likely
problem. That usually requires a pretty impatient goldsmith to
quench it that quickly on this sort of open, light weight design.
contamination with metals like lead, in assembly of the
mounting. Such low melting metals can penatrate the white gold,
leaving the metal weak. And some white gold solders can also be
somewhat brittle, so if this is near where some assembly too
place, and such solders were used, then improper assembly could
be the culprit.
Maybe you just have a bad head. heads are nice clean looking
stampings, sure. But they’re made from sheet metal, and that has
to come from somewhere. It’s not common, but a flaw in that
sheet metal, like a small bubble, might be undetected, yet lead
to a head with the starts of a crack. The fact that two prongs
simultaneously broke would fit this scenario, since a larger
flaw in the sheet could easily bridge two or more prongs in the
stamping. For the record, though, this is pretty rare. Usually,
bad sheet metal leads to stampings that fail before the stamping
process is complete.
It’s also possible that improper setting/tightening techniques
are at fault, though this seems less likely with inside prongs.
If the prongs were repeatedly bent back and for, just a little,
they might not show marks, yet could have been weakened.
Hope this helps.