How to Choose the Perfect October Birthstone As Halloween approaches, the excitement builds, with elaborate decorations and spirited costumes igniting creativity in all of us. It’s a time to celebrate not only the whimsical aspects of the holiday but also the deeper connections with the October Birthstones, and October’s main birthstone is Opal. Opal is a stone known for its play-of-color effect. Basically, it is a mineraloid, meaning it has no ordered atomic structure and is made up of hydrated silica and water. The water content of opal can range from 3–21% by weight but is usually between 6–10%.