Black opals

Jewelry Discussion
1

G’day I noticed that one person on this list gave
instructions for turning certain types of opal black, using
sulphuric acid and sugar. Had you realised that the blackening
in this case is carbon? Heaps of years ago I had to do lecture
demonstrations, and one was to show the power of concentrated
sulphuric acid to remove not only water from a material, but to
even remove the ELEMENTS of water (X H20). Sugar (sucrose) is
C12 H22 011 so you can see that if you could remove 11 molecules
of H20 you would be left with 12 of carbon. So add concentrated
sulphuric acid to a small amount of strong sugar solution.
Nothing seems to happen and the 250 first year chemistry students
start to giggle, ready to burst into roars of derisive mirth.
Then with a hiss and a roar of it’s own the reaction starts;
sending clouds of steam and sulphur dioxide to the ceiling, and
when all the hero spectacle dies away one sees that the 2 litre
beaker and the large bowl it stood in is filled with a steaming
black mess of porous carbon. Which is (more or less) what
happens when the opals are so treated. The trick can also be
done with certain agates, to blacken them. Onyx, anyone?

    /\
   / /    John Burgess, 
  / /
 / //\    @John_Burgess2
/ / \ \

/ (___)
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2 
  G'day    I noticed that one person on this list gave
instructions for turning certain types of opal black, using
sulphuric acid and sugar. Had you realised that the blackening
in this case is carbon?

Correcto, John. While studying for my G.G. from G.I.A., we also
learned of another treatment for opals was to wrap the stones up
in dung (not sure what type works the best) in a brown paper sack
and slowly cook it in a campfire! :slight_smile: I guess this is another
form of carbon, huh? I just love modern technology! LOL Ken