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Test for coral


#1

Hello all you wonderful people, I feel like I have the most simple
questions sometimes but I don’t have any where else to turn to. I was
grubbing through a ‘junk’ shop’ which was labeled an antique store.
Wild exaggeration I must say. But I found a string of what I feel is
coral chunks. Quite big pieces and lovely color. I asked where did
it come from but was given some story about a local beach town which I
am sure does not have coral. I looked at it very closely and it
seemed to have all the traits of a growth, coral item. But knowing
all the effort put into ‘fake’s’ here and in other S.E.Asian
countries I must do some test to prove to myself that its real. If so
I can continue with my designed (in my mind) necklace. The only info
of testing I have found so far is " drop a bit of acid onto the
surface and it will bubble" Excuse me, but this does not tell me
enough. What acid and how long and… so on.

Thank you very very much for your help.

Sharron in Saigon, waiting for the next six weeks of school to finish
and get on with the move!!!


#2

Hi Sharron, Coral should have a specific gravity of 2.6 - 2.7 and a
hardness of about 3 - 4 on the Mohs scale. Most imitations (especially
from a junk shop) will fail either or both of these tests. Specific
gravity can be tested by rigging up most any decent mechanical scale
over a container of water. Let me know if you need better
instructions!

All the best,

Dave
Dave Sebaste
Sebaste Studio and
Carolina Artisans’ Gallery
Charlotte, NC (USA)
dave@sebaste.com


#3

Sharron, destructive test are not the best way to go. The acid will
not tell you that it is coral only that is is not. There is a lot of
things that will effervesces to hydrochloric acid.

The test for coral are

  1. Refractive index 1.486 to 1.658 with a Birefringece of 0.172.
    While the RI covers a wide range, the Birefringece is diagnostic on a
    spot reading.

  2. Cavities from Polyps and a wavy Fibrous structure

  3. Specific Gravity of 2.65 plus or minus 0.05

4 Polish luster is waxy to vitreous

5 Fracture luster is dull

6 Fracture is splintery to uneven.

Check under magnification for evidence of dying. This will be areas
of concentrated color especially along edges and cracks.

The best imitation is the Gilson Coral which has a RI of 1.55 with a
weak Birefringence and a SG of 2.44. These two test will separate
this imitation from the natural.

Hope this helps. Don