As some of the most unique and special gemstones in the world, opal are known for their iridescent, fire-like appearance. If you look at opals from a historical context, you will find that throughout centuries, people considered that opals were made up of multiple gemstones and they were a single gem that was a cluster of different stones. This was because of the fact that many opals had radiant patterns and color combinations within them. These people believed that the spots of red were the ruby, the spots of green were the emeralds, and so on and so forth.