The color of blue agate is because of the interaction of cryptocrystalline quartz with trace minerals like chromium and iron. The blue agate stone within the blue agate jewelry is polished in order to showcase its colors and patterns in the best way possible. The chemical formula of blue agate is SiO2. On the Moh’s Scale of Mineral Hardness, blue agate ranks between 6.5 and 7, which is why you can consider it to be a fairly hard stone.