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Opals and ultrasonic cleaning

Hi Folks,

recent discussion about ultrasonic cleaning had a large number of
replies about not cleaning opals with an ultrasonic cleaner because
they will crack.

Which has got me curious. Why would opals crack in an ultrasonic
cleaner? are there other materials that also crack, and is there a
common theme?

Note I’m not saying I disbelieve, I’m all for empirical evidence -
i. e. you tried it and it broke…

what I would like to understand is why it happens - i. e. the
physical mechanism - and what other similar materials crack in
ultrasonic baths…

perhaps we can discover a way to prevent opals cracking in a

(N. B. I studied materials engineering and geology, so I have a
pretty good grasp of the physical properties of materials,
crystallography, microstructures & etc.)

Dr Paul van den Bergen

Dr. Paul

Opals are like sponges, they will definitely dry out!!! They need
to be well lubricated in storage, at all times.
You should put your
opal into a ultra-sonic cleaning machine. then you can study the
after-effects under a strong microscope. You will see the latice-work
of the cracks being then formed. Why? You are removing all of the
oils. not a pleasant thing to see.

Opals are not a crystal!!!

I once worked for a jewellery company & they forgot to keep their
large selection of Opals well oiled. every one of them had severe
cracks in them. None of them could be used, pity!!! Gerry

Paul- I failed to pay much attention when I was in school so I can
only offer the School of Hard Knocks here. The list of items I have
personally seen destroyed by an ultrasonic cleaner.

Opals, pearls, lapis, malachite, coral, fracture filled rubies, and
sapphires, dyed jade, turquoise, emeralds, and rhinestones. There
are so many treatments of colored stones and diamonds out there.
Pretty much most of the colored stones out there have been treated in
one way or another and they are most susceptible to damage from

It is not only the action of the ultra sonic cleaners but also a
combination of that, heat and the harsh detergents needed to remove
polishing compounds and decades of packed in dirt under stones that
have been worn daily.

I have also seen about a dozen loose 10 point diamonds destroyed in
the sonic by someone who placed all of the stones in a small basket
together for cleaning. Between the sonic and steamer he chipped all
of them because they rattled up against each other.

Have fun and make but don’t break lots of jewelry.

Jo Haemer

Hi all

Opals are not crystals, they are spheres of silica if I remember

This is why they will break in an ultra sonic.

all the best

Hi All,

What type of oil is used to oil opals? Denny Diamond


On the other hand - counter intuitively materials with LOTS of
cracks are very tough - e. g. pearls…

Hi folks,

First off, I can start to see a common theme for the sorts of
materials that ultrasonic cleaners are contra-indicated for - they
all have microscopic cracks - thanks Jo for that list.

Some have extensive microscopic voids or cracks - or lamella - like
malachite and pearls, some may have a few microcracks.

Some are surprising - emerald and sapphire for example. it may not
be as simple as microcracks though, the shape of the crack and the
brittleness of the crystal (surface free energy -

may play as large a role as anything…

wrt the opinion that opals are not crystalline, this is wrong - or,
well, misleading…

For starters, there are various “degrees of order” between
crystalline and truly amorphous materials - opals are somewhere along
that spectrum, made more complicated because it is has a microscopic
structure as well as an atomic structure (most “crystals” have atomic
structure only) - the spheres that make up opal are crystalline - but
not “fullly” crystalline - they have a higher degree of disorder than
say quartz. The matrix that infills the space between the spheres is

Regardless, it’s not the amorphousness that is the problem. Can you
clean glass beads? if so, being non-crystalline is not the problem
nor a common theme

nor is being semi-crystalline - c. f. chalcedony and agate - both
structurally and chemically similar to opals.

The treatment of opals with oil and their ability to gain and lose
water may play a role, especially if there is shape change involved.

Heat may play a role - once again especially if heat causes volume
changes - or loss of water and associated volume changes. especially
shrinkage which leads to easy crack propagation.

My gut feeling is that the combination of low toughness materials
(surface free energy) plus microcracks, aggravated by any treatments
or enhancements (that are reversed or interact with the cleaning
solvent or cavitation, causing surface shrinkage) are the key

oh boy. don’t google “gemstone energy”… depressing

the link to the CSIRO research included an interesting image that
demonstrates what I’m thinking quite nicely…

here the surface of an opal has been etched with HF in a process
normally designed to remove some of the amorphous matrix
hydroxy-silica to enhance the appearance of the stacked silica
spheres in the electron microscope - instead the acid has caused
exfoliation of the surface due to significant changes in the volume
of the material and consequent cracking and spalling of the
material. The other place I’ve seen this sort of behaviour is in
rusting steel - where the curling is in the opposite direction due to
the iron oxide occupying considerably more volume than the original

I would use light dabs of “Mineral Oil”, as this semi-liquid is not
too thick. and it won’t evaporate.


What type of oil is used to oil opals? Denny Diamond 

What kind of opal? Most opal should not be oiled. The only exception
I’ve ever heard of is sugar-and-acid treated opal which has been
subsequently turned hazy by mistreatment.

Al Balmer

I don’t put big nice opals in the ultrasonic but regular sized ones
I do and have no problem. Most opals in for ring repairs are not
super spectacular. They are more susceptible to heat and cold

Years ago like 30+ we used to use mineral oil. worked fine then.
Know some say no. but it does help to keep some opals that are not
stable ferromagneticdrying out and cracking. Just my experience.

Way back when we used glycerin on opals as well. Had nothing to do
with cleaning and the ultrasonic but the mineral itself actually
dries over time. Oil or something like it is used as a sealer.

I would use light dabs of "Mineral Oil", as this semi-liquid is
not too thick. and it won't evaporate. 

My father always stored his opal rough and finished stones and even
finished jewelry with mounted opals in glycerin. That’s about all I
know about it. Rob

Rob Meixner

What kind of opal? Most opal should not be oiled. The only
exception I've ever heard of is sugar-and-acid treated opal which
has been subsequently turned hazy by mistreatment. 

My opal is 40 years old; a 10 carat carved beauty framed in green
gold, withsix 15 pt. diamonds. It was my wedding present from my
husband. It’s wornon special occasions, so it isn’t constantly
exposed to the environment. I keep it in a drawer in a fairly
humidified room.

So, do I or don’t I oil it?


My opal is 40 years old 

If it’s good after 40 years, why on earth would you want to mess
with it? Is it Australian?

also which says “The stones
should never be kept in oil or any other chemicals. This may cause
them to lose some or all of their fire.”

Al Balmer

Hi all

the Aboriginal opal miner I know keeps his rough in water.

Also I have been told Aboriginals oil opals by rubbing the stone on
their foreheads to get the body oil into the stone.

Also they can be set and the setting polished as soon as the setting
gets warm stop and let it cool down.

Opals seem tougher than you would think. My daughter swapped 3
silver and stone rings I made her for an opal doublet I was given by a
miner set in 18 kt yellow band with a silver bezel. I like the
combination of metals.

Yes just another easy push over dad.

She wears it all the time and in all conditions and it has survived
for over three years, much to my surprise.

Out of all the family jewellery I kept, the rest was well not worth
keeping, are a ring with 3 solid harlequin opals and diamonds. And a
genuine scarab bought by my grandfather when he was an engineer on
the Egyptian railways and lived in a train carriage. It is set
traditionally in fine gold and looks like it will fall out anytime
don’t know if my granny ever wore it.

The scarab will be left to my son and the opals to my daughter.

On jewellery not worth keeping why do so many women have lots of
jewellery of little value than a few good pieces? Sentimental

Also I do not understand why women get replicants of expensive
jewellery and keep the real one in a bank vault.

If you can’t tell the difference why bother?

Here is another weird thing I really like making jewellery and have
done for decades. But I cannot stand to wear it.

I some times make something for my self but cannot wear it for more
than a few hours before I put it into stock. I do wear a watch
though. Go figure.

all the best

Thank you, Al, for the info. link. Very interesting.

I believe my opal is Australian. You are right, I may as well not
mess withit.

Many thanks,