The opal was named opalus by the Romans, meaning “precious stone,” and it’s easy to see why. The color play in opals, also known as ‘fire,’ are part of what makes opal so valuable, and is caused by the regular arrangement of silica spheres inside the stone. The spheres cause the diffraction of light that we see as colors. Opals are deposited at low temperatures (400 degrees F) in circulating, silica-bearing waters. Because of the chemical makeup of opals (SiO2*nH2O), the amorphous growth, and water content, opals are not considered minerals, but instead mineraloids. Some opals are more crystalline, but they are considered less valuable than the amorphous opals. Opals come in all colors and formations, but they are colorless when pure. Common opal, known as 'potch,’ has no color play and is typically a solid, opaque color. Fire opals (pictured above) have reds and yellows in them, but typically no color play. Wood opals (pictured above) are formed when opal replaces the organic parts of wood. Red color play in a black opal is considered the rarest form of color play, while green on white is the most common. Opals also come in nodules, stalactic masses, veinlets, and encrustations. They make up important parts of sedimentary buildup, such as diatomaceous earth. Opals were even considered to bring good luck according to medieval lore, because they had all the colors of the other gems inside them. They were also believed to aid invisibility, and are the October birthstone. Thank you to dayscorpion for the suggestion!