Onyx is read as self-control and firm boundaries — a dark stone that keeps its own counsel. Some dream books also see in it a grief or seriousness being carried with composure.
Does the traditional reading fit your dream, or did it mean something else to you?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.