Tourmaline comes in many colours, sometimes several in one crystal, and is read as balance between different parts of yourself. Black tourmaline in particular is traditionally seen as a shield.
When this appeared in your dream, what were you feeling?
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.