A solid opal is a natural opal that exists as a single stone throughout its entire thickness, without any additional layers or reinforcements.
A doublet consists of two layers: a thin layer of genuine opal bonded to a base of a sturdy material such as ironstone or black obsidian. This strengthens the opal and makes its colors stand out more. A triplet consists of three layers: a thin layer of opal glued to a dark base and topped with a clear quartz or glass cover. This top layer protects the opal and enhances its shine and colors.
A mosaic opal is a special type of opal that consists of many small, flat pieces of genuine opal glued to an often dark base like a mosaic. The mosaic opal is then often covered with a clear protective layer of quartz or glass, similar to a triplet.
Each of these variants has its own unique characteristics and offers different aesthetic possibilities. More details and pictures can be found in this post .