Sorry to be late getting to this, my wife has been in the hospital
and my mind has been there rather than on things like this.
Let me offer a different view on the hows and whys cracks in opal. I
do want to emphases this is speculation on my part, I don't have the
equipment to verify it.
There has been some interesting work done on the effect of moisture
on cracks in glass. A limited review was published in "The Eclectic
I believe the original article was:
B. C. Bunker and T. A. Michalske, "Effect of surface corrosion on
glass fracture," in Fracture Mechanics of Ceramics, edited by R. C.
Bradt, A. G. Evans, D. P. H. Hasselman et al. (Plenum Press, New
York, 1986), Vol. 8, pp. 391-411 If anyone has access to the original
I would like to read it.
The proposed fracture mechanism is that when a crack occurs in glass
and moisture is present, a chemical reaction takes place at the
bottom of the crack causing the crack to grow. If you want a better
explanation, read the references, I am trying to put a long article
into one sentence.
The effect appears to take place in obsidian (a natural glass), but
as far as I can find, no work has been done on whether quartz is
affected by the reaction.
Now for the speculation, if opal is susceptible to the reaction,
then any opal that has been scratched or has had its surface polish
abraded (or wasn't well polished to begin with) could have cracks or
crazing occur over an extended period of time. Opal's molecules are
more tightly bonded than glass so if the reaction does take place, I
would expect it to be slower and sometimes taking years to occur
doesn't seem unreasonable to me. I would also expect that opals from
different locations could give faster, slower, or no reaction. I
would also expect that this would be a different mechanism and
independent of the traditional moisture related cracking normally
associated with opal.
If anyone has had unexpected crazing of finished stones it would be
interesting to know if they could have had surface problems prior to
the occurrence. It would also be a good argument for getting the best
polish possible on your stones.