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                  A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Opal:

Most opal is 50-65 million years' old, dating reverse to the time of dinosaurs in the Cretaceous period. Opal created as silica from decomposing rocks that assorted with ground water and formed a silica gel which hardened in underground cavities and fissures.

 

Opaque:

In gemstone use, an opaque gem is one which does not allow light to pass through it, is not transparent or translucent.

 

Onyx:

A variety of chalcedony quartz, frequently with parallel banding. When chalcedony is variegated it is called agate. Black onyx might be opaque or translucent and though it does occur obviously in black, it is usually dyed to darken its color.