Opal is the result of seasonal rains that poured over dry grounds, leaving solid deposits of silica in cracks that eventually form into opal. There are several different categories of opal based on color, such as fire opal (which comes in brown, yellow, orange or red, with red being the most valuable). Although fire opal does not have a play-of-color characteristic, most opal does. This is often seen as a flashing rainbow or sparkle. Throughout history, the gem has been recognized as the luckiest of all gems. Opal is a birthstone for October.