This criterion only applies to those species which are transparent. It also considers varying degrees of transparency from “water clear” to opaque, within the context of what is known about the specific species. Any species not commonly found clear is far more valuable in crystals that are. The commonly used term “gem” or “gem-quality” implies the piece has a gem like appearance, which is the highest form of transparency and the most sought after in all species. The property of a solid object being transparent has for millennium intrigued and fascinated humans. We are drawn to objects that glow. The light shining through a crystal should create a luminescent glow.