Since it is composed of calcium carbonate, coral will effervesce if touched with acid. Imitation coral is made form glass, porcelain, or plastic. Aluminum is often anodized, as is magnesium, titanium, and tantalum. Most obsidian is 70 percent silica. Obsidian has a hardness of 5 and a specific gravity of 2.35.
Some apatite stones show a cat's eye asterism .
This stone is rarely used in jewelry because it is brittle and soft. Apatite has a hardness of 5 and a specific gravity of 3.15-3.22. The bangle above is reverse-carved and painted in the areas that are carved. In a process called called vapor deposition, the quartz is put into a vacuum chamber and attached to very hot electrodes. Today, blue aquamarines are more highly valued, but this was not true in the past, when sea-green stones were prized.