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
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
Cavities from Polyps and a wavy Fibrous structure
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