Gemstones that Begin with "C" Gemstone Crystal System Density Hardness Refractive index Treatments Carnelian micrcrystalline 2.65 7.0 1.564-1.660 dying, filling Chalcedony micrcrystalline 2.65 7.0 1.564-1.660 dying Chrysoberyl orthorhombic 3.72 8.5 1.746-1.755 none Chrysocolla microcrystalline 2.00-2.24 2-4 1.54 none Chrysoprase microcrystalline 2.65 7 1.564-1.660 dying Citrine hexagonal 2.66 7.0 1.544-1.553 heat Coral organic 2.65 3.5-4.0 1.486-1.658 dying Corundum trigonal 4.00 9.0 1.762-1.770 heat, fracture filling