Hi Chris, unless you inadvertently omitted a key word or two of
the reply, shame on that GG for not giving you a better response!
Without more details or seeing the piece, I can’t say if he was
right or wrong… and please read all my reply, so you know why I
say that! GIA does teach a bit more than a paragraph on testing
coral, and in fact, coral is presented in several forms in the
practical part of the gem ident course, and in the colored stones
course too. I’m not a GG yet, but on the way, and here’s what I can
tell you about testing coral:
First, coral can have a polish luster that ranges from waxy to
vitreous… so the look of the finish is not proof either way.
(And there are no standards for the ‘sound’ test, so we have to
rule out your ring as a test instrument :>)
Natural coral can have cavities from polyps and glass as you
mentioned can have surface pits or cavities, so that alone is not
diagnostic… it just tells you that you need a closer look. You
don’t mention if either of you used magnification and what you
found if you did.
Magnification is the key test for coral… does it show the
typical wavy, fibrous structure of natural coral? Does it show
swirl lines, mold marks, or gas bubbles of glass? Does it show the
fine granular structure of Gilson imitation coral?
We’ve already determined that the presence of surface cavities
alone is not diagnostic in this case, but are there any chips to
show a fracture shape and luster? Coral has a splintery or uneven
fracture that is dull; glass has a conchoidal fracture that is
Specific gravity can be diagnostic if it is under 2.60 or over
2.70 – the range for natural coral. Glass overlaps this range and
can run from 2.30 to 4.50, so you can rule out natural coral if the
reading is outside nat. coral’s range. If it falls within, it
doesn’t prove anything. Note also, imitation coral has a Specific
Gravity of 2.44, that with a fine granular structure is fairly
strong evidence of its identity.
So in your specific case, the structure under magnification would
have given you the answer, and even though it seemed ‘glassy’, it
could have been natural coral if it showed the typical fibrous
structure. (And it is distinctive!)
IMO, the GG should have told you the Refractive Index 1.486-1.658
(for natural coral), the specific gravity (between 2.60 and 2.70)
and about the structure under magnification, and that a vitreous
polish luster might not be common, but is found in natural coral.
Then you would have known why he said it was indeed coral… if it
was! HTH more than it confuses things! Carol
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