Coral vs Glass

Carol_J_Bova · May 15, 1998, 1:42am

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
vitreous (glassy!)

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

Rev_Dodwil · May 16, 1998, 6:01am

I don’t know if it was mentioned, but since coral may be in a form
that does not exhibit many if any of the visual characteristics
needed to identify it, you will get a birefringence blink on the
refractometer. A lot of corals (calcarious) are calcium-carbonates
and their strong birefringence give a great blink on the
refractometer, with a polaroid lens. The black corals (conchiolin)
will not.

Later,

Arthur

P.S. I like run on sentences

Chrissy · May 16, 1998, 4:17am

Hi Carol

Thanks for the info! my Refractometer is broken and you would have
thought he would measure it, specific gravity test would be hard
to do since it is already set into a pendant and I am sure he did
not want to do it and didn’t sound like he was a bench jeweler, I
can tell that the swirl lines are found sometimes in coral and in
glass, I have about 20 pieces and sometimes you can see
irregularities in the coral and sometimes not, but the pit markes I
was talking about under magnification were all union in size and
all over the back which is indicative of probably air bubbles when
the glass was molded and the Shine probably was because they did
not polish it off to a satin finsih it was a mold shine! It would
be comparable to taking a vase that is glad molded out of the mold
it comes out shiney and if you want a satin finish you would have
to polish it to that.

Although scientifically there is no credability to the sound test,
I did it with other pieces in my collection and the faux piece I
had just purchased definitely had a very glassy sound to it! And
you can’t consider the setting I guess which was a cheap silver
setting, because years ago I guess they could have set a nice dark
coral in a non gold setting, (not a usual)

What I did was put knowledge and Common Sense into my evaluation
to come up with the answer I did and felt very strongly about it ,
he did refund my money in full! You have brought up a good point
why didn’t he test it with his refractometer.

Thanks again! And good luck with your degree, I have been going to
jem shows since I was 18 so must of my knowledge came out of books
and asking a lot of questions!

Kind Regards
Chris- Sunny and beautiful in CT.
http://www.tace.com/glitters