1. Place cumin seeds in a small dry skillet, and cook over medium heat, stirring constantly, about 3 minutes or until lightly toasted and fragrant. Transfer to a small plate; cool. Crush cumin seeds in mortar and pestle. (Or place in a heavy-duty zip-top bag, and crush with a rolling pin.)
2. Combine cumin, olives, and next 5 ingredients in a shallow airtight container, stirring to coat. Cover and refrigerate at least 24 hours or up to 1 week, stirring occasionally. Bring to room temperature before serving. Garnish, if desired.