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The Queen is an alchemical term used for their mercurial water. The King is the Sulfur, who needs to get married to the Queen. The king has a red coat because he represents the third phase of Redness. The Queen has a white dress, and thus represents the second stage of Whiteness. As she is central in this picture and on a podium, this image here shows the second state of Albedo or Whiteness. Furthermore so, because above her is a moon, also a symbol for Albedo.
Sometimes she is named by the alchemists as the Greek goddess Diane.