Stones that are Blue

Gemstone Crystal System Density Hardness Refractive index Treatments Agate microcrystalline 2.66 6.5-7.0 1.544-1.553 dying Aquamarine hexagonal 2.72 7.5-8.0 1.577-1.583 heat Azurite monoclinic 3.7-3.8 3.5-4.0 1.730-1.838 none Benitoite hexagonal 3.64 6.0-6.5 1.757-1.804 none Chrysocolla microcrystalline 2.00-2.24 2-4 1.54 none Corundum trigonal 4.00 9.0 1.762-1.770 heat Diamond cubic 3.52 10 2.417 radiation, drilling, fracture filling Lapis Lazuli various 2.75 5.50 1.500 none Moonstone monoclinic 2.56 6.0-6.5 1.518-1.526 none Sapphire hexagonal 4.00 9 1.762-1.770 heat, fracture filling Spinel cubic 3.60 8 1.718 none Tanzanite orthorhombic 3.30 6.0-7.0 1.690-1.700 heat Topaz orthorhombic 3.53 8.0 1.619-1.627 blue is irradiated Tourmaline trigonal 3.06 7.0-7.5 1.624-1.644 heat Turquoise triclinic 2.73 5.0-6.0 1.610-1.650 dying, sealing Zircon tetragonal 4.00 6.50 1.810-1.815 none