Opal

Opal is the modern October birthstone and the accepted gem for the 13th wedding anniversary. Most opals are between 50-65 million years old, dating back to the Cretaceous period when dinosaurs roamed the earth. Opal formed as silica from decomposing rocks mixed with ground water which formed a silica gel that collected and hardened in underground cavities and fissures.
There are two distinct types of opal, common and precious. The way the silica particles form determines which type. In precious opal, silica particles are packed in regular rows and layers. …