Iron and rocks containing iron show yellowish rust due to

Iron gets its rust from exposure to moist and oxygen

Iron–rich rocks in thin section Magnetite and hematite are opaque under the microscope under transmitted light. Under reflected light, magnetite shows up as metallic and a silver or black color. Hematite will be a more reddish-yellow color. Pyrite is seen as opaque, a yellow-gold color, and metallic.