Most Agates are found as nodules in volcanic rocks; but could be found in various rock formations. They are common in metamorphic rocks [rocks changed profoundly chemically and/or physically by temperatures greater than 150 Celsius, about 270 Fahrenheit.] The photo shows a 1/8.5 inch slice of a Geode Brazilian Banded Agate. The shot was taken outdoors using the sun as light source. The slice was hold directly against the lens of the camera. The camera then pointed to the sun. Sony DSC-H9, F-stop f/8, exposure 1/3200 sec, ISO 100, focal 5 mm, Aperture 2.875